ProtonDrive is the only cloud storage option with a free tier recommended by privacyguides.org.

Another option like others have said is encrypt your data first and you can use any service. You can create a veracrypt container and just upload that file. I read a comment somewhere that dropbox is capable of handling encrypted containers so you don’t have to re-upload a veracrypt file several GBs in size every time. I haven’t tried that yet though.