Thanks everybody. The Sharkbite has been installed. Getting the old compression fit valve undone was, as I predicted, not fun because there’s just no space to work wrenches, but once that was out, the prep and installation was easy.
I can sweat copper old school style and I have PEX tools as well, but this was just one of those one-off situations where I really, really, really wanted the easy fix rather than have to remove the toilet and make a bigger project out of this.
I already have another project to move on to. Both my MIL and my sister have townhomes that were built in the late 1980s/early 90s, so absolutely everything that wears out is at the end of its usable life. Now the hot water valve on the MIL’s laundry shutoff is dripping a little… So that’ll mean the cold water is shot as well, so let’s just replace everything… New valves, new hoses, new drain hose.