I'm with you on the upgrades. Plain old bonkers some of them.
I think change the feet and the mat, maybe the external cables - I've yet to to do mine - and then basically find the right cart.
I went with an ATF3 in the end. They just marry together really well, each part counteracting the flaws of the other.
A Pickering XV15 625e was a delightful combination - nice open sound, lovely & clear, lots of detail but also real nice balance across high, mid and low tones. It was so good I donated it to my friends SL1210 set up which I bought her as a graduation present a few months ago.
My Nagaoka MP20 also works excellently on an SL1210, but the ATF3 is just that little 'better' sounding to my ears.
If an SP25 is as good as a SP10 then yeah it would be one to get. Lovely design too.
All told there are 5 more belt drives to try for me if I can - Logic DM101, STD305, Michell Gyrodec SE, Nottingham Analogue Spacedeck and a Townsend Rock. Then I'll definitely, absolutely call it a day on the experimentation with BD's.
For idlers like you I would like to have a listen with a 301 or 401 and also the hybrid belt & idler TD124.
However none of these are pressing and I will most likely get all my current stock of TT's (11?) tweaked, modded & set up and be content.