I originally intended to just upgrade the SSD, but when I opened the bottom case I found the batteries to be swollen (and there to be a ton of dust and cat hair). I first ordered the battery adhesive removal kit and removed the swollen battery. Once that went well, I grabbed the thermal paste replacement kit as well as a new battery to do a complete refresh.
The battery replacement was actually pretty simple: going slow and letting the adhesive remover do it's job made things pretty straightforward. I felt that removing this battery was actually easier than removing my iPhone X battery.
I wish there was more info on the thermal paste application for my specific machine, but it was easy enough to find info elsewhere. Cleaning out the fan and replacing the thermal paste was pretty straight forward with the ArtiClean remover and purifier.
Finally, it wasn't an iFixit repair, but I also upgraded the SSD from 128GB factory stick to a 500GB NVMe SSD.
The machine is now running fast, cool, and quiet, with hopefully several more years of life ahead of it.
I've done a fair bit of repairs, but as I've taken repair more seriously, having a good set of tools and a well lit/clean workspace has made "complex" repairs like this one much more manageable. Having the tools and space has made me more confident in seemingly difficult operations.