I use a custom domain with iCloud+. That works for me because I'm very much in the Apple ecosystem, but I can easily move somewhere else whenever it's no longer the best option for me, and I did just that when I moved from Proton to iCloud. I've set this up for my wife as well. It can be as simple or as hard as you want it to be, but above all, I'd strongly encourage you to use a domain you own. Email has become our de facto identity, and we should be in control of it.
+1 to this. I was using Fastmail and it was very good, but since I'm already paying for Apple One, I decided to switch my e-mail (in my own domain) to iCloud+ to.
The only con I see is: there is no way to add separate 'mailboxes' to it. I can register multiple addresses, but it's still one account, thus one mailbox.
What if you lapse domain renewal for some reason and someone buys it? Some serious accident or life event? I do the same but i use icloud as main one.
you can prepay many domains for up to 10 years.