I've been doing this for a while but the part I haven't managed to solve is that using your own sync server on Firefox for ios seems to require you to also run your own Firefox accounts server - which is much more painful to set up. Is there a similar easy guide for setting that up?

Try this one: https://mozilla-services.readthedocs.io/en/latest/howtos/run... It's not as detailed as for sync, but has all the links to very detailed guides (even for a docker-based one). Hopefully it helps.

upd: typo