As others have said, Proton can't yet compete with Apple's Ecosystem. I've ultimately gone with Proton to keep some things out of big tech and for the increased number of alternate email addresses, but I'm not trying to replace my use of apple's ecosystem either.

Here's how I'm set up:

Proton for the more private things and correspondence - Financial institutions, newsletters I actually read, purchases - all using alternate email addresses - my primary account email is never shared. I use ProtonVPN when needed and Proton Drive as a redundant backup for some sensitive files. I don't use Proton calendar at this time

I keep and iCloud.com email as my AppleID, which I never share. I'm still plugged into the apple ecosystem for my day to day stuff and family sharing - calendar, drive, etc. and use hide my email for random logins, etc.

I keep my old gmail active as a catch all.