OP's suggestions are great, especially the family organization part.
One other thing I would highlight: spam filtering, particularly blocking modern scam and phishing emails that come from hacked accounts and websites and not just the "obvious" sketchy providers and spoofed addresses. Anyone who has a long term email address probably knows how hard this is even for the biggest players. The inbox of my old gmail that I created during the beta is nothing but scams and phishing now. For my personal domain, however much I wanted to like Fastmail, the spam filtering was so useless that I gave up and switched to M365. Masked emails for each account can help shut off the barrage from an email that's been in a database dump, but they're not a panacea.
Spam in my inbox is mostly just annoying to me (I have low tolerance for any kind of unwanted notifications), but if Kagi Mail was going to be something I'd feel good about recommending to less tech/security savvy friends and family, I especially wouldn't want the protection against phishing to be an afterthought.
As for what good protection looks like in implementation, I don't have any great ideas off the top of my head, just saying it's a priority for me.
