I love doing legacy content but it’s not really engaging with you 1 shot everything at 70!
Would turning off Exp anywhere from lvl 30 - 60 make sense if all I want to do is old expansion zones/questing with that character? What level would be most appropriate?
If you’re in Chromie Time, I would turn it off at 57. You should be able to do just about everything in any expansion (other than Dragonflight) without it being trivially easy, and you can still change expansions if you want to. (I believe at 58 you’re locked out of changing your Chromie Time selection.) (BTW, I read a comment on Wowhead that if you freeze XP while in Chromie Time it will kick you out of it, but you can talk to Chromie to get back in again.)
If you’re not in Chromie Time, it would depend on the level cap for that particular expansion. Check the zone map for the range of levels applicable to the expansion you’re in.
If you want to do stuff in order, I’d lock at 49 until you’ve finished Mists of Pandaria to minimize phasing issues that BfA can cause. If you don’t mind doing BfA stuff earlier to fix up phasing issues, then you should be okay to lock at 59, but 57 is probably safer. (There were bugs early on in Dragonflight with Chromie Time at 58/59 and while I haven’t seen any recent issues, why risk it.) Also be aware that once you hit 60, you can’t turn off XP any more. (Nor can you access Chromie Time.)