That’s what I would do if I had to start from scratch or had to change disks. But as the disks are way more than 50% full, I cannot easily convert the media backup to borg.
Also, I am currently using czkawka to compare my full sd cards with the remote backup to ensure I have the photographs backed up before removing them from the SD card. This is a bit more complicated and probably slower with borg fuse mount (but not impossible). Other methods may be possible, such as adding the contents of the card entirely to the borg repo, comparing if there are files with the exact same chunk list, and removing these files from both the card and the snapshot. But I would probably stick to option 1.