Upgradability of an MMF-5

I'm aware that there are turntables that are more upgrade-friendly but really want to zero in on this table exclusively for this discussion. What upgrades/tweaks, etc, to this table do you think would make an instantly obvious and moderately to highly gratifying sonic improvement in a system limited to the MMF-5 running thru a 1990s Sony ES series HT receiver running thru JBL L-65 speakers (circa 1977). Any and all types of music in a collection of about 500 LPs but with the lion's share being mainstream Rock (not a lot of Metal), Jazz, and Blues...
If you have the stock Goldring cart on there, get something different.
I have a relatively new MMF-5.1 and have not considered upgrading other than a Herbies mat just yet. When I do I will address the isolation with a solid wood base and substitute spikes in place of the rubber feet. Later a cart and possibly look in to fitting one of Projects carbon fiber arms. The cart and phono stage you choose will probably make the biggest difference for you.
As Ehaller said, changing the cart with yield results.

I would not recommend spending further dollars on any other upgrades to this table.
Invest in a new turntable/tonearm.

The MMF-5 is a "buy and listen to without changes" product.

Sell it when you want to upgrade.
I think a Koetsu Tiger Eye cartridge would help.
Otherwise do what Mr. Feil says, just get a better rig.
I changed the stock stylus (1012) to the 1042 stylus and the difference was huge, more weighty sound, bigger sound stage, less surface noise...
I've had the MMF-5 for 4 years now. Definitely change the cart, other than that just leave it be. I think its not worth upgrading, the money is better spent towards a new table. I've tried lots of upgrades and found ultimately that the weaknesses of the table persist, and do so to a degree that its just not worth it.

A fine table for what its worth, but not worth upgrading in the long run.