About a decade ago, this worked great. All I had to do was create a symlink within my Dropbox and the directories were backed up.
Then a few years ago, Dropbox stopped supporting symlinks because they “caused problems”. It’s unclear what problems exactly because I’ve never actually heard of anyone who experienced any issues. Nonetheless, the solution was to move the real directories into Dropbox and put a symlink in their original location. This _did_ cause problems, including needing to disable SIP, and breaking navigation in Finder which treats symlinks like shortcuts.
Finally, the latest update completely broke backups for me. Admittedly this was not Dropbox’s fault; they were forced by Apple to use a new API which doesn’t allow the Photos library to be backed up.
I moved my backups to Backblaze B2 and cancelled my Dropbox subscription. Now I use rsync to back up my machine, and utilise Dropbox’s free tier for the small amount of cloud storage I actually need.
Oh, and for the amount of data I back up, B2 is almost an order of magnitude cheaper.