I'm in the same boat as you – happy with iCloud as a host for personal domain, but absolutely hate the client. That's part of why we're building Marco: https://marcoapp.io I'm building it for myself, but sounds like it could be a good fit for you too.
Marco seems really interesting! I'm curious as to how the UX is with solely IMAP. Whenever I use iCloud/Outlook/Gmail accounts on a different email client (e.g., using my Outlook email on Mail.app or my Gmail on the Outlook client), the IMAP experience just feels subpar compared to using the regular client (I have three email clients on my dock right now because of this...). Usually this shows in not great push notif…
We have spent quite a lot of time and thought creating a label abstraction that makes _all_ email accounts play together nicely. There is currently no other product like this on the market.