I guess I don't fully understand what is your use case but I think you're over-complicating things.
If you only care about some files being encrypted, you can just encrypt those while leaving the rest of the system untouched. This will make the file transfer much easier and secure, even across online services. You can use something like Cryptomator for that:
On the other hand, if your use case is to encrypt most of your files except for a few of them, you can just use any file sharing service. I'd recommend this which would encrypt the file in-transit anyway: