Looks like some combination of defragging & balancing has done the trick! The space that was previously marked UNREACHABLE is now UNUSED, and my disk space is back to normal:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       227G  105G  103G  51% /

Thanks for the wiki link, Btrfs is new to me and I’ve definitely got some learning to do

Legend! It found a second filesystem named “UNREACHABLE”:

It looks like an exact duplicate of my main filesystem “/@rootfs”, I’m guessing this is why my disk space filled up. Do you know how I’d go about removing the duplicate? (If it’s safe to do so)

Yep it’s not near finished. This is one of those projects that’s sat in my “keep an eye on it” bookmarks for a good while, I figured I’d post it to get some attention on it, because it does look very promising.