For years I was using Boxcryptor, but they have decided to go subscription based.
The basic requirements are Mac and Windows client, but also some way to access the files from Android and IOS.
Having single files and not a file vault that all the files live inside is very important. This help with cloud sync of the encrypted system. Rathe synchronise a bunch of small files, than one big one.
And so we have Cryptomator.
Free client-side encryption for your cloud files.
Open source software: No backdoors, no registration.
Now, we cannot determine if it is really open and no backdoors, but having a Github library with all the source code ( https://github.com/cryptomator ) does go a long way at making it trustworthy.
Give it a try.