I have installed Dropbox on Zorin 17.1 and it seems to be working fine. However, I want to add a subfolder within that dropbox folder and encrypt all files that go into it. I am also syncing dropbox to a Windows PC (and another linux machine) and need the files to be available on all of them.
There is Cryptomator, you might want to check it out and see if it suits your needs. This thread may be a bit out of date, so make sure to contrast it with the official documentation.
This seems to work well. However, the only issue I have with it is that I can't work out how to get it to startup when I login. Having to start it manually is slightly frustrating.
I'm not sure how to do this either, I've never tried to set it up this way.
The only reference I found that can be useful is using one of their plugins to use KeePassXC as a backend for your passwords. It looks good; once setup, it should decrypt automatically the vault you are trying to access. But I suppose KeePassXC needs to be running already with an unencrypted database.
KeePassXC is a password manager, btw. If you already use it then it won't be an additional step in your setup.