Cartridge upgrade, something warmer

It's not the cartridge.

Elizabeth's guidance is definitely the first thing to try. Then adjustments to your arm/cart setup. Then cheaper tweaks like TT mats (felt is horrible, though slippage wouln't usually result in thinness), clamps, etc. All as advised above.

That said, some Arcam gear can be thin sounding (owned their top CDP, sold it) and all SS gear can except at very high levels. Even there it is when compared to comparably priced tube gear. That's a path worth exploring but I'd do the cheaper stuff first. Everthing you learn will train your ears and inform your decision-making ability when it comes to costlier upgrades. You'll make fewer mis-steps.

Again, it's not the cartridge. A Grado would provide lots of artificial warmth, but Grados on Rega rigs have a long history of annoying their owners. If you must experiment with warm sounding carts I'd try some MM's. Not much money and Raul and the MM crowd could advise on suitable choices.

Some good advice posted on here.
To which can I add; try to find an original ARC Ref phono for its warm, tubey sounds and cartridge wise add a Koetsu Rosewood to your collection.

Are you using the PE mode on the P75? I had that combo for a couple of years and it never sounded thin to me. I found the P75 to be lacking something with other carts, but in PE mode with the 20XL it was wonderful.

Now, if you are in PE mode I would suggest what Elizabeth recommended and try to go lower on you settings on the P75.

Wow so much great info and suggestions. I'll start by making changes in the P75.
Thanks to everyone.