I've been trying to switch to iCloud email today, but apparently you need to set up an @icloud.com email address before you can use your own domain, and every single thing I've tried I get back a "This address is taken" message, even with entirely random strings [0]. They also have a 21 character limit, just to make it harder.
[0] https://twitter.com/drcongo/status/1484489810932862982