From what I understand, Tuta is cheaper than proton and can give him all the compartmentalization emails he needs, and it doesn't need google for notifs (I think?). He uses Mullvad so no need for protonvpn, and Tuta has their own calendar, too.
He can switch his google voice numbers over to Jabber and access them with their Cheogram app. It's a bit of a pain to set up but its been working well for me.
I am reluctant to recommend silent.link because of a poor experience with them, but they are the only truly non-kyc ESIM provider that I know of. If you set him up with them, I suggest only giving them $5 at a time, go into the esim settings and manually select the network, and if it starts bugging and disconnecting, re-install the SIM (if possible, pay attention to his SL ESIM info page), and don't have them transfer money between ESIMs.
He can also use You Have Mail to get notifications for proton. Its Polling, not Push, but setting it to check every 5 minutes generally gets the job done for me, even for janky fake "2FA" emails.
I have done some learning about Native Alpha... This hasn't been updated in a while but because it uses webview, it is as secure as GOS's Vanadium webview. As best as I can tell, at its core, its just a roundabout way to add bookmarks to your home screen that have their own barebones vanadium windows instead of opening in new tabs. It has some other doodads but that's pretty much it. No notifications and you gotta pay attention to details in the settings to make sure things work, but I have been having good luck with it so far. I use it for discord (while I figure out RethibkDNS so I can safely use the app and see if I can't get notifs without google), proton email and calendar (while I migrate to Tuta), and a banking website, and will be using it for a lot more.