Great write up!
So your family is ok and happy with using aliases but didn’t want to change providers? I’ve only used 3rd party clients and IMAP so the concept of the provider being important is foreign to me.
I’ve been slowly moving business and personal email accounts to hosted, using Hover and Namecheap, and the experience has been ok. I only use 1 alias for online accounts, just to keep things simple. I expect it will be years before I stop receiving email at gmail. I’ve had a custom email for several months now and the wife has zero interest. Same as her using a password manager other than Keychain.
How do you like Runbox? In my research to find the perfect family email hosting provider, I somehow missed it. My kids aren’t old enough to have an email account yet, but they will very soon (actually my son now has a MS account for Minecraft). I’d like for them to live in a world where they don’t have an address at every free provider.
My family members were happy to use their respective Google and Outlook accounts. What I mean by that is over the years, one tends to use the Gmail account to log in other places in addition to google services (photos, youtube, maps etc). For that reason, they do not want to move away from a service they are using for years to a service (namely Runbox, Fastmail etc) that they have never used. For this reason, I found this solution where they can continue using their preferred service. I use third party apps too but my family uses the default Gmail and Outlook apps ( I suspect most people do this). I already see the benefit of this change, where they now give out their firstname@surname.com email address.
Tip for families: get them into a service early on because its very difficult to get them to move later on 
Runbox is very good and reliable service. If for some reason, you are not happy with Fastmail, please do consider Runbox. It may come down to interface, ui design but if you use third party apps, this is not much of an issue.