$2000 To Upgrade System

Looking at your system, I would say in order of importance, upgrade the speakers first, then the phonostage/cartridge, and finally the integrated amp.

I would say speakers/amplifier first, cartridge/phonostage second. And possibly cables too. Another approach would be to start with the cartridge/phonostage and then work your way downstream. In any case, it appears that your turntable is the strongest component.

I wouldn't have said this a few years ago, but how about a integrated with a inboard phono ? It would have to be from a manufacturer with a history with analog. A Creek and Rega passed through my hands awhile back when I was inbetween gear. They were good performers and the phonostages were not an afterthought. A few less connections in the chain and a better power supply (in some cases) also.
Look for used (recent models) here for that and take your time shopping for speakers after. That really can be a personal thing and you would have a solid foundation to evaluate behind them.
Just another way to look at it.

I think you're fine with the turntable and speakers you've got. I'm not a big fan of Bellari phono stages, and I haven't heard the Marantz unit. If it were me, I'd start by auditioning amplifier options. The Naim Nait 5i with a Rega Fono MM would fit your budget and be worth a comparative audition to see if that's a direction you like. I think it would make a good fit with your Rega front end and the well-timed, well-balanced Wharfedales.

My first impulse would be to change the cartridge. $500-600 will buy a very nice cartridge. Then spend the rest on speakers.

I think I'll go the amplifier/speaker route first. I'm a relative newbie at this, but I'm not sure how much an upgraded phono pre or cartridge would help the sound when played through my budget amp/speakers. I realize my TT is outclassing everything else in my system so I feel like I'm not hearing it's full potential. Any speaker/amp suggestions in this price range?

We don't know what kind of sound you prefer.
Naim/Devore, Plinius/Proac, Creek/Spendor ?

just my op but consider room acoustics, cables and vibration/resonance control first.this way you could discover more about the gear you have before spending big bucks on one area. i bet your system sounds pretty good already. what music do you like and exactly what are you trying to achieve with this upgrade?