I am thinking about setting up a discourse instance at *.scheme.org

It is open-source, can work as a mailing list, and can be self-hosted.
Also it is possible to import mailman archives.

Usually, people use one of those subdomain:

- discuss.scheme.org
- forums.scheme.org
- discourse.scheme.org

Which one do you prefer?

Personally, I prefer to ask upstream to host it in exchange for
$USD100/month with 50% discount when officially non-profit.

ref: https://www.discourse.org/pricing