How to find all files that were modified since last backup with borgbackup?

I backup all my files (with borgbackup) every 2-3 months to an external harddisk I store off-site. The problem I want to solve is how to protect the files that changed since the last backup. These are probably not big and could be stored on a free tier cloud storage. I was thinking: if should be possible to find all files that were changed since the last backup (maybe with find *all files newer than the file I touched after the last backup*), create a hardlink to a folder on my harddisk and backup this folder to the cloud. Am I forgetting something? How would such a script look like? Is there a simpler way?