I agree about a change of TT first.
When I upgraded from a Thorens TD166 BC with an RB250 arm to a Nottingham Analogue ACE Spacedeck with an RB330 arm, I kept my Goldring 1042 cartridge, and despite not changing the cartridge there was a hugely noticeable uplift in sound quality, including loss of distortion and improved (but not overblown) bass weight. The benefit of improved pitch control and tracking, for instance, cannot be overstated. At present, I have no desire to change cartridges as I love the sound that I'm getting, but I also know that I now have a proper platform for doing this should I ever change my mind.
Have you considered upgrading your 1012 to 1042 next time you replace the stylus? That would achieve an improvement in sound. (I started out with a 1012 and worked my way up to a 1022 and then a 1042. Each upgrade was very worthwhile.) Might be an alternative to going MC, unless there is something seriously pushing you towards MC. For me and my ears, the TT upgrade also showed me just how good the 1042 is.
Regarding SUT versus various preamp options, perhaps your ears might help.
The main practical problems that I can see for you right now are: -
1) There are too many variables right now, and you need at least one stake in the ground. A new TT might be a good move whilst, perhaps, keeping the RB300.
2) Finding a dealer to fully support your journey might be problematic inasmuch as turntable and cartridge auditions are potentially the most costly and complex things for an audio dealer to support and manage. If this leaves you relying in part upon the advice of others, the best advice is likely to come from someone who really understands what floats your listening boat. Others might inadvertently offer advice that better floats their boat than yours.
Finally, if you did opt to replace your TT then you would need to consider how it would be used. For instance, would you just listen to LPs, or would you flip between 33 and 45 and therefore need easy speed changes? Are you a TT tweaker who does on-the-fly upgrades and loves to realign their cartridge, or do you prefer plug and play? Etc.
If it's just about getting at the music in the grooves then the best TT will be the one that you're likely to use the most. A super duper TT that's too complex for you in terms of use and maintenance could easily become just an expensive piece of furniture that gets dusted but not used.
(I did this with a bad telescope purchase. I went for an expensive Celestron Nexstar Evolution 8" Cassegrain telescope with computerised Go-To functionality etc. but found the set-up etc. too much of a faff; and there it sat. Meanwhile, I'd already donated my old Orion Europa 6" reflector with its basic equatorial mount that was so easy to carry out and point at the sky. Big mistake!)