They were friends I had sex with/was attracted to, but they were friends nevertheless. We talk about interesting, important things, we hang out. I don't want to abandon them again. I think I'd like some advice here on if being friends with past fwb while youre in a relationship is possible?

For some people, it is possible. However, I wouldn't say it's the case for a majority of people.... especially if you've still had sex with them until very shortly.

Sometimes I think that the only situation that would work for me is polygamy. I can really support and care for and talk to multiple people, without the pressure of falling in love and exclusivity.

You might talk to people within the community. Often people in that scene are married/in relationships and still have a primary partner. Often they also have boundaries in place, so you can't just do whatever.

Should I go for it, but risk feeling disappointed and hurting him?

I think you should not pursue this guy. He's told you what he wants. Instead, you should work on yourself and maybe talk to your school counselor to work through your past relationship. Polygamy isn't an easy way out to not deal with your eventual feelings. If love scares you, you should figure out why etc. This is something a counselor can help you.