I think I now hoave a minimal set of steps required to get Mastodon dev running on an M1 Macbook Air. I may have missed some, because I got there through a bunch of trial and error, which may have installed extra dependencies. If this doesn't work, please let me know.
These instructions are based around https://docs.joinmastodon.org/dev/setup/#vagrant and https://github.com/ppggff/vagrant-qemu .