Namely, de-facto, or one of, in Linux. Mature. No GUI. Open-source and free.
What is it? GPG or anything else?
For a separate file(s), or directory(ies), and not for the entire disk or partition.
If you use ext4 or other filesystem that supports fscrypt, you can use fscrypt to encrypt specific directories.
There’s also gocryptfs for a fuse-based userspace implementation.
ZFS has built-in encryption: https://klarasystems.com/articles/openzfs-native-encryption/
I don’t want to encypt them in-place because I’ll be uploading them onto a server, copying them on an external drive.
I’ve been using gocryptfs now for a few years and it works fine as you describe.
You initiate the encrypted folder, set up automatic backups for it. Then whenever you want to access it you mount it into another folder.
There is a distinction here between the permanently encrypted folder that you can upload backup whatever, and your temporary mount, unencrypted folder.
If you’re alright with the rare conflicts to fix yourself something like syncthing works well for this setup even across computers.
This.
Thanks to Meta BTRFS is apparently got/getting it to a certain extent too: https://youtu.be/6YIc2fVLVPU?si=ngiHWS0fw2zIHf2M
