Remove duplicate Mojave from partitioned drive

Because of a disk error on my boot partition, I used Internet recovery to temporarily install Mojave on my data partitition, so I could clone the malfunctioning drive (it was giving an fsroot error) with Carbon Copy Cloner to an external drive, and wipe the data partition, and reclone back to the boot partition.

(It was longer than I was comfortable since I'd backed up to TimeMachine)

So.....how do I uninstall MacOS on the data partition, without entirely wiping it? That's a whole new level of pain because of how large it is vs my backup drives, and risk of losing data (mostly photos) if I get something wrong). I don't remember any uninstall option anymore in the installer, because you're never supposed to need to do this, but directories do get corrupted, and I'm worried that just deleting the system folders will leave behind a system component that can't be deleted and will lead to future crashes. Plus: I need that space for working with photos. I've been taking too many while stuck at home.