looking for advice to mitigate inner groove distortion

I currently have a Nagaoka MP200 on a rega P3 with all the groovetracer upgrades (including counterweight for better tracking).  I've calibrated the cart using the Stevenson method but I am plagued by inner groove distortion.  I'm considering getting a MP500 with the line contact diamond to help.  Is this a good idea?  I know IGD is part of the deal with Vinyl but I get bummed out every time the last song on the record comes on.
Are you absolutely positive all your adjustments are as good as you can get them?....are you sure, the arm is has free motion....not hitting/rubbing on anything?
I too have an MP200 (along with several other cartridges all premounted on headshells) and experience absolutely no inner groove distortion. Something must be off on your alignment or adjustments.
I would re-align the cartridge, maybe using other methods, and check everything else. This should not happen as you describe it. I would examine the cartridge too.
Audible IGD is not necessarily "part of the deal" with vinyl.  (Tracking angle error is typically greatest at the innermost grooves but this is not necessarily linearly related to audio signal distortion.) Can you describe what you hear and when you hear it?  As in, where is the stylus in relation to the innermost grooves of a typical LP when the distortion sets in?  Is it related to actual tracking or to the distance between the stylus and the spindle?  If the latter, look for something that is obstructing the travel of the tonearm, like a wire that is getting fouled or a problem with anti-skate or with your tonearm bearing.
I hear it mostly in horns and vocals.  It's a fuzzy halo around the sound of the instrument.  Also in loud passages of orchestral works that occur in the inner grooves.  I start to hear it about 3/4 of the way from the edge of the record to the end of the grooves and it gets worse as it gets closer to the center.  It's worse on some records than others and sometimes, I think it's the record that's the culprit but it seems that the turntable and cartridge are playing a role as well.  I checked for anything obstructing the travel of the tonearm but I didn't find anything.  I tried adjusting the anti-skate.  Some fine-tuning helped a little (maybe, not sure I trust my ears sometimes) but it was still there 
What tools are you using to ensure that your cartridge alignment and calibration are correct?
Both channels?  Or one channel more than the other?
cleeds, I think your thought about alignment is reasonable, but as was noted elsewhere, misalignment is not usually that obvious in my own experience and that of others.

Also, where is the motor located on the Rega P3?  Is it under the platter or outboard and perpendicular to a line drawn from the pivot to spindle?
cleeds, I used an alignment protractor and my eyeballs :)  I used a tracking force gauge to calibrate my tracking force.  The anti-skate on the P3 is pretty primitive so that part is imprecise,

lewm, both channels and the motor is located under the platter in the center and on the rear side of the plinth
I used an alignment protractor and my eyeballs ...
Not all cartridge alignment contractors are equally precise.  Many align the cartridge itself, which isn't ideal. The best alignment gauges, imo, align the cantilever. They're much trickier to use, but I think worth the effort.
It sounds like you’re using one of the freebie alignment printouts?
Some of those are okay for alignment only after having set overhang.
How did you set overhang?

Overhang must be set accurately first. Otherwise if overhang is off it will cause your cartridge to be rotated too much one way or the other, which you may not notice until the inner grooves where it becomes more obvious.

My favorite for being fast, easy and accurate, is the MoFi Geo-Disc  https://www.mofi.com/product-p/mfslgd.htm which you will notice has a ridge line running A to B which must be aimed directly at the tone arm spindle vertical axis. Then you tape the platter so it doesn’t move while you’re tweaking alignment.

Even though you are not using the Geo-Disc nothing changes. The Stevenson you’re using must still be placed accurately in relation to the tone arm spindle, and both the gauge and the platter must be held stationary.

Try again with these tips and overhang set first and see how it goes.

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https://www.analogplanet.com/content/uni-din-versus-l%C3%B6fgren-b-just-clarify according to Mikey's findings,  Stevenson alignment has the most distortion most of the time. Unless you're playing very long records you will experience the most distortion near the end of any album side.
Don’t forget to clean your stylus regularly too. A dirty stylus can seem okay outside the inner grooves.
Greg, the saving grace of Stevenson alignment is low distortion at inner grooves, compared to the two popular alternatives. One could argue about alignment all day but I strongly doubt that misalignment or Stevenson alignment can account for this problem. I’m now wondering whether motor is interacting with the cartridge as the two get closer to each other. To test this, with the platter spinning, pick up the tonearm and holding it above the platter but close as possible to it bring it over toward the spindle with volume control fully advanced.
Yeah lewm I also can't think of any alignment issue that puts a halo around horns. While I would like to nail down the overhang situation the halo and crescendo comment sounds more like mis-tracking to me. 

Adam, maybe double-check VTF and if not max maybe try a little more.
I also suspect mistracking ... bit if it is then there may we’ll be groove damage and in that case nothing will fix it. Is this just on one album, or on many, including brand new ones?
Just get $100 new cartridge and try it with it. We can't see or hear what you do. It's inexpensive experiment.
One could argue about alignment all day but I strongly doubt that misalignment or Stevenson alignment can account for this problem.
It's much easier to get phono cartridge alignment wrong than it is to get it right. If LPs exhibit inner groove distortion, alignment is the first place to look if you want to be methodical. You're likely to be just chasing your tail if you don't first confirm that the alignment is correct.

A distortion analyzer can easily show the results of a misaligned phono cartridge.
Cleeds, I don’t think we are listening to each other. I am saying that I do not think that the particular symptom reported by the OP is due to misalignment. I did not say that misalignment could not cause distortion, although I have my private doubts about the audibility of such distortion. You say that misalignment can be detected on a distortion analyzer. Do you know of any published data to that effect? I would be interested to read any article about that, but I have been unable to find such an article after doing a computer search. By the way, all three of the popular alignment algorithms result in some degree of tracking angle error at the inner grooves, as you probably know. Of the three, the innermost null point for the Stevenson alignment is closest to the spindle. For that reason inner groove distortion with the Stevenson alignment is probably not worse or significantly worse than that of any of the others.But this is a minor point. I agree with you that most of us have made a tiny error here and there in aligning our cartridges. That means nearly none of us is listening to a perfectly aligned cartridge.

I think that the OP might be experiencing an effect of the Rega motor on his cartridge due to EMI. The fact that it gets worse as the cartridge approaches the spindle is consistent with that, just as it is consistent with inner groove distortion. Furthermore, it seems to me that I recall that Rega motors are known to interfere with some cartridges, in particular Grado cartridges, as I recall. But I could be wrong. Checking out the role of the motor in this phenomenon is actually easier than checking tonearm alignment.
IME, inner groove distortion can be caused by many things.  Not sure how old your cartridge is, could be the suspension is worn and you need a new cartridge, or in your case a stylus replacement.  Could be the way the diamond is cut does not like your stevenson alignment.  Try a new stylus. 
Apparently the OP is on vacation.
tz, You have successfully come up with an explanation for the OP’s problem that is even more unlikely or far-fetched than my own or any of the others. Are there any data suggesting that stylus shape and alignment algorithm are linked in any way? If so, I am willing and anxious to learn about it.
Well now lewm don't be so hasty. He also said it could be the suspension. He already gave it an alignment. Maybe just needs new shocks?
IME, which is many years BTW, certain cartridges and stylus profiles do track different and better than others. YMMv but that is my experience.  Not far fetched, just my experience.  my Pickering tracks better than my Grace or my Benz's.  Could be the suspension, compliance or stylus profiles do have something to do with it and I am not sure what kind of QC some of these manufacturers have regarding diamonds and the quality of them.