What would the requirements be in the case I wanted to *permanently* teach in Japan? (cross-post suggested by r/JETProgramme, apologies if this doesn’t fit or is a stupid question)

Sorry if this doesn't fit here, since it's more of a question about after the JET programme.

I'm about to go into college for my first year this coming Autumn, and want to go into teaching under the JET prog after. I'm getting an Engineering degree, because I know it's nonspecific what type of bachelor's degree you need, and it could be useful. But I want to make sure it's not gonna screw me over in the long run.

I'm not certain if I'll feel the same by the time I'm done with college or JET, but I'm passionate about the idea of teaching. I know it sounds dumb because it's a stressful, thankless, underpaying job, but I am.

Say I finish the programme and loved teaching, wanting to fully expatriate, move to Japan and become a permanent teacher. What would that look like in terms of requirements? Would I absolutely 100% need an English degree? Would they be less forgiving of me keeping my hair long or having pierced ears? Would it even be possible to get employed as a foreigner trying to become a teacher while not in any programme?

Apologies for the stupid question, I'm just curious.