Next step up from my Rega P3

At that price range I would take a good listen the P7. From what I have heard at the dealer is that TT is great value when compared to the P9. At about half the price of the P9 the P7 offers 75-80% of the sound quality. I think Rega offers turntables that are way under rated for the price point.

I agree that upgrading the phonostage and cartridge probably makes the most sense with your budget. I'd probably recommend a modified EAR834P for about $2K. That will leave you with $1K for a cartridge. The Rega P3 is nice player and you should notice the improvements. If you want to take your table to the next level I'd recommend getting out of the Rega line and trying something like a Nottingham.

As others pointed out, you haven't fully exploited the Rega's capabilities. Both the wiring and counterweight upgrades make noticeable improvements to an RB-300 arm. I kept my modified Rega arm and used it on a Thorens TD-850, with good results. Since there are 4 or 5 elements to this -arm, table, phono pre, cartridge, sometimes phono cable - you may be better off picking one weak point to upgrade, like the phono stage. That way you will be able to identify the source of any change in sound. You might as well take your time, since as you know, from reading this forum, it will never end.

At $3K, I'd go for a new Well Tempered Amadeus. If the reviews are even half
true, it would be a step or more up from the P3 in every way.

Firebaugh has the track record, and now he's got an affordable and consistent
source for manufacturing.

I would check this WT Amadeus, too.