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

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

Proton has good service

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 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.

ICloud. Although, it doesn’t really make any sense if you’re not already somewhat in the “Apple ecosystem”.

Last I checked it’s e2e encrypted.

A more general suggestion is probably ProtonDrive or NordLocker. I’ve used both of them in the past but found them slow and clunky, tbf it was some time ago and they were still in beta.