Thanks a lot for the extensive reply!
Based on your experiences (and also what @Ed_S shared), it seems that even the CAX21 might already work. And for a smaller project that I’m currently spinning up in parallel, the CAX11 should do (didn’t expect it to outperform the CX22 so clearly).
I’m not sure whether I will use mail-receiver, but I do agree that it seems like an omission, given that the rest of the stack and also most other things one might need for a Discourse install (Traefik, Crowdsec, etc.) seem to be compatible with ARM64.