Will a phono pre-amp make a substantial difference

@fritzenheimer  I don't have your NAD, nor do I have any experience with the Rega phono. That said, according to AI it describes the sound of the two as follows: 

"The Rega Fono MM MK5 is a dedicated external phono stage known for a dynamic, punchy, and musical sound with excellent detail and a wide soundstage

. In contrast, the NAD M33 has a very capable built-in phono stage, but its sonic signature is more restrained, often described as warmer, cleaner, and slightly veiled compared to the Fono MM's detail and excitement. For a vinyl enthusiast seeking the most engaging and detailed vinyl playback, the Rega Fono MM MK5 is likely the superior choice, while the NAD M33's built-in stage provides a more balanced and laid-back sound that some may prefer. 

Does the descriptive of the NAD match that of your listening experience with it?  If so, do you like that presentation ,or would you prefer the sound as AI describes for the Rega?  If unsure, maybe consider calling say Upscale Audio and discuss with them as they carry both Rega and NAD; and potentially if there's a different/better recommendation for your sound preference within your budget.

I changed my phono from a Schiit Skoll to a vintage Accuphase C-220. The difference totally worth it. 

However, at least in my case, this comes with some draw backs, as the C-220 is quite old and even if it was recapped, still has some issues which I am currently trying to fix.

Depending on your budget and whole system, go and buy the best phono which you will find it. Ideally, try and test a few in your system, to hear the differences and what you like.

I would definitely go for the Fono MM MK5 and I especially like your choice of the 2MR Bronze with your P3 which in it’s original iteration has been around for a very long time and is highly regarded, also with the fine line stylus will track and render groove information better with lower noise than many cartridges in it’s price range. You certainly have been very thoughtful about your source and amplification choices and getting the most bang for your buck. Way to go!

Take a look at the Rega Couple 3 interconnect for each of your connections to your amp. They are made with Klotz cable and Neutrik connectors and are very well assembled and will be good enough to use in any future upgrade to your system no matter what level you decide to take it to. I believe that’s what’s on your P3. Food for thought.

It is also worth looking into how the TT is supported?

How effective is the present mounting able to Isolate the TT from being effected by local ambient energies being transferred within the listening environment.

The stylus creates a mechanical energy that is converted to being and electrical based signal to be amplified / gain added.

The Phonostage is extremely important, firstly for the basic reason it is the first device to add gain to the conversion to the electrical signal.

A correctly thought out mounting methodology for the TT, will substantially assist with the Sent Signal being extracted info from the LP only. Hence, the gain added by the Phonostage is to a signal sent not containing energies / minimised unwanted energies that create an adulterated signal.

A Phonostage is a crucial device in developing the Sources sent signal to become an End Sound. The Phon' has a much much improved influence when enabled to process what is a minimal adulterated to pure sent signal.

I stand strong in the notion the expensive to purchase Phon' receiving a poor produced sent signal, is a less attractive End Sound, than a cheaper Phon' receiving a Signal that is close to being unadulterated.