> First, I tried mailbox.org, which I can generally recommend without reservation. Unfortunately, you can’t send emails from any address on your own domain without a workaround

I use mailbox for a long time, one account for 2.50EUR/month with multiple custom domains and I can send emails from any address. To send from a different address the process didn't really seem different than other providers.

From Thunderbird mobile on Android I just add a new sender identity. If I need to send from webmail, similarly I just add a new alternative sender. Are these the workarounds you mentioned?