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Proton has good service

I would suggest an alternative that allows you to use any of the “Big Ones” like iCloud/OneDrive/GoogleCloud/ Dropbox.

Buy Cryptomator on whatever device(s) need access and set up a single vault on any cloud service you use. This vault will be client-side end to end encrypted which means its just you with knowledge of and managing the encryption keys. Put them in your password manager for a while until you memorize.

Remember, if they aren’t YOUR keys, it doesn’t really belong exclusively to you.

Proton Drive: https://drive.proton.me/

Tresorit is the best of them all.

It has all the features that matter, it’s fast, it’s E2EE, have automatic photo upload, it’s cross-platform … ticks all the boxes.

It also ticks the high price box, though.

Besides that you have Filen (good but not great, very cheap for a cloud storage with E2EE), Skiff Drive (lacks some features, but it will be my preferred cloud storage as soon as they have automatic photo upload), Proton Drive (lacks a lot of features and it’s expensive), MEGA (I have a legacy account, it’s 50GB that I use for the non-sensitive data), …

I’ve heard good things about backblaze which has an s3 compatible API (meaning it supports most tools). Like s3 this probably means it has a similar learning curve