For me it’s Whatsapp. Like 99% of people i know use it so yeh… i still need it. I am considering moving the Whatsapp Gdrive backup to a local backup btw.
Google photos is also a major problem for us. We have a lot of stuff that want to migrate to different service (or store locally).
I’ve been with Posteo almost a decade, that part was pretty easy. I remember downloading everything from Google Drive was pretty easy back then as well.
Then I used stock android until maybe a year ago when my phone was too old to get updates and too old to be supported by my banks app so I migrated to LineageOS.
The messaging app landscape is kind of a mess, so I use SMS/MMS BC as you’ve noted the landscape is fragmented at best.
I solved file storage with AlainKnaff/Android-SFTP-Documents-Provider which just SFTPs into my desktop at home.