Instead of u/username its @username@instancedomain which translates to https://instancedomain/u/username
Instead of r/subname its !communityname@instancedomain which translates to https://instancedomain/c/communityname
Basically, in the body of a post or comment, use the ! or @ format to create a workable link.
Vorta for Borg Backup - for linux and MacOS. You use it remotely but I use it for local backup because a) its encrypted b) its Borg so awesome and c) easy to use. I just pointed it at my home directory, told it where to place the encrypted backups and how often to make them.
I've had to recover files twice and recovery is just as easy as set up.