Hello and welcome. You sound like an interesting, motivated person. I’d start by checking out this thread and really giving it some thought:
More specifically to you, you don’t mention whether you’ll need FA, or if you’re Korean (or other ORM) or otherwise. Per the linked thread those things will matter. But I like the focus in your ECs in terms of the mental health and awareness stuff. If you really lean into that it could make for an interesting essay if you can find a truly personal connection to it or personal motivation beyond wanting to “make a change in a stress-building world.” Something that only you could say or bring to the table.
As far as Harkness, the truth is that most BS offer small classes held in a discussion format, whether officially Harkness or not. Exeter is explicitly HARKNESS for every class. Andover uses Harkness-style roundtable discussions for some classes but not for others (true of most BS). You can find more detailed info about this on schools’ websites.
You mention having a good local option, which is important. Know that many, many boarding schools offer excellent academics beyond just the tippy top ones you may have heard of (or that I mentioned…) Think about the other aspect too. Large vs small, rural/remote vs close to/in town, single gender, % of day students, etc. All of these things would impact your experience in some way and so should be factored in at least somewhat.
Try to take the SSAT unless truly prohibitive to do so. I know many schools are nominally test optional right now, but unless your IB program is somewhat known your grades may be somewhat tougher to calibrate.
Hopefully others will chime in. Good luck to you!