What I have found so far:

• posteo looks good, but does not support custom domains
• tutanota can support it though I somewhat doubt their survivability long-term
• proton mail also supports it, but not a huge fan
• zoho: tried it years ago and it was ok, subprocesor list has aws and google on the periphery but not core
• ghandi.net: ?
• fastmail: ?
• kolab: ?
• Mailfence

Are there any others to look at?

I am not looking to self-host.