Phono Stage, Cartridge, DeGritter...which one first?

Sell the P6. Based  on reviews, the P8 is a significant upgrade from the P6. Forget additional tweaking of the P6.

To be "all in" Rega camp, P8/10 with their Apheta/Aphelion cart. You can continue the madness after that by further upgrading phonostage. By then, you discover the rest of the system is not keeping pace, and wondering if you should jump ship and try another brand table/cart.

Just figure out a budget, get the best cart/table and phonostage. Until you've invested at least $5K-10K+or so, a US cleaner isn't necessary, IMO. Buy good records to start, and you can postpone a US cleaner. I've done this for years, and get away with a $75 Spin Clean.

The proof is hearing  the very same records and playing them on uber systems, and hearing them how they were meant to be-noise free, organic sounding and dynamic as no file or CD can match.

I only buy used LP's, so my saying buy "good" LPs to start is a challenge. That's another thread.