If I end the preseason mid-gold...

Assuming you have the appropriate MMR, I'd expect to end up around mid-Silver.

Probably between Silver 4 and Silver 2.

It'd depend on where you ended the main season at much more, as preseason only slightly impacts your mmr going into placements.

If you ended the main season as a high diamond MMR, but flopped to mid gold MMR in preseason, you'd probably still place low-mid gold.

If you ended the season as a mid silver MMR but climbed to mid gold MMR in preseason, you'd probably place mid silver-ish.

If you ended the season in the depths of bronze 5 MMR, but climbed to mid gold mmr in preseason, you might place low silver perhaps.

The exact details of the soft reset aren't publicly known (and neither are our MMR's for that matter), so these are just estimates based roughly on previous seasons, but it's really hard to pin down anything exact. It's even possible for two people with the same end of season mmr and same end of preseason mmr to do exactly the same in their placement matches and still get placed in slightly different divisions (although alrge differences are unlikely) due to randomly getting matchmade with slightly higher or slightly lower MMR people, resulting in slightly different final MMRs.

Still, the most important thing to keep in mind is end of season MMR matters much more than end of preseason MMR. Improvements to the latter are still helpful, but you should probably have your expectations in line with ~1 tier below your end of season rank, and then maybe give yourself an extra division for every 2-3 divisions you climbed in preseason. It's quite possible that'll be an underestimate, but better have your expectations in line and potentially be pleasantly surprised.

Well Division is literally meaningless for placements. Placements take into account your MMR, not your rank. You can be Gold 3 with Silver 2 MMR. You can also be Gold 3 with Platinum MMR. Division is meaningless besides for end of season rewards