Perhaps unsurprisingly, the ideal phono stage for a Rega cartridge is a Rega. I’ve no idea about SUTs so can’t advise on that one. If you want to keep things simple, my first thought is that a Planar 8 with Exact should be a lot better than a 6 with an Ania Pro. That’s what I used with my Supernait 3’s internal stage and it was excellent. On the other hand, if you wish to stick with the 6 and Ania Pro, a Fono MC might be a value choice. The Aria is quite expensive, but very good, and would stand you in good stead if you think you may later get a better turntable.
With your Rega deck and MC cart, I doubt you will get more synergy from anything else than a Fono MC or Aria phono stage.
IMO the only other option would be to go MM, trade in the Aria Pro for something like an Ortofon 2M Black directly into the Nait XS3, but given the price of the Fono MC why bother?