You're young enough for majors and should shoot for legacy/SWA. No question about that. If for no other reason than you won't have a job if you don't have an FO, and if legacies have to hire CFI's (math says they might) then there won't be many CFI's at regionals.

The only legit question is... what happens if I get stuck? In that case some things are under your control, others are not...

You can pick a regional which has base(s) that you like. Base(s) plural is better, since bases come and go.

You can choose a regional which is generally known for being "less toxic" than others, and generally has good QOL (although QOL is often totally variable depending base/seniority).

If legacies don't pan out, then look at LCC's. That will depend on their growth, contracts, and other issues at the time so there's no right answer on that today. Might want to do LCC, or might be better off staying at a regional. It will depend on where things stand at the time.

If hired at a legacy by age 50, you should upgrade and still have plenty of time to progress (retirements). Or you can hang out as a senior FO and still make good money (significantly better than RJ CA). For example... regionals might match 5% on the 401k, legacies will typically contribute 15% ish. Contribute, not match.

Bottom line, regional lifer, you won't know if it's a good idea until your 65th birthday. Especially given the uncertainty of the future of the business model, you'll be along for the ride and in suspense the whole way.

Bad things can happen to majors too, but you'll typically see it coming a long way off.