Is remastered mainly just less jitter?

OTOH, the really early masters for CD are poor - if one were to avoid listening before buying maybe the 1990s would be the safest bet.

Re-mastering can use newer, better technologies; can remove some artefacts from say wow & flutter, can degrade the sound with compression, and can reflect personal preferences of the engineer or fads

Jitter is likely the least problem or benefit in re-mastering

"Dark Side of the Moon ... there is one VERY famous remaster that totally kills it dynamically and sound effects-wise"

is that the one Barry Diament did?

also the 2002 or so re-masters of the Stones 1980s releases are supposed to be MUCH better

Agreed - it is a  crap-shoot,  crap shoot, or at least a  crapshoot.

Find a ratings list, then see if you can try before you buy - if not be prepared to re-sell the crappy ones, or shoot at them as targets