That spot welder was exactly what I needed. I know it's a bit of a 'duh' question given how simple repacks generally are in theory, but I don't think I would have considered it otherwise based on what I was finding on Google, since I didn't realize you could get them that cheaply nowadays... So thanks again for that!
14650 cells are bit harder to come by and what I could find online was a little odd since the Hitachi factory cells were 3.6V but 14650s seem to all be 3.7V nowadays, I ended up taking a risk on some generic looking "AMT" 3.7V flat top cells from an eBay listing and it turns out they worked just fine. The biggest challenge of the whole thing was mostly just re-attaching the weird right-angled controller PCB
(which involved getting a soldering iron a little close to the cells the way I was doing it
) and getting the right length/width on the nickel strips, they're a little wide so I ended up cutting them in half down the middle which gave a much better fit.
I covered most bare metal I could see with electrical tape just out of paranoia which resulted in something a little bulky that didn't seal perfectly when you put the top case back on, but even in two pieces it will still hold together enough to go back into the tablet and lock just fine with no bulging or otherwise badly fitted look on the back. Tried just turning the tablet on after and it worked! I'm planning to use it as sort of a mini-terminal/desktop on a stand with a mouse and keyboard but now I can take it wherever I want...
The battery seems to charge properly with the AC adapter and the charging light will turn from orange to green after a while which gives me hope that the overcharge protection circuit is still working with these new batteries, but ResInfo doesn't seem to be able to get the voltage levels or any other figures from the battery controller. Is that really concerning at all or normal for repacks? With that aforementioned use case I'm hoping to still leave it plugged in for long times and preferably unattended, but I don't know if the overcharge prevention is trustworthy now...
In any case, thanks for the assistance! Excited to try kludging up my dead Clio and Aero 8000 batteries with this thing next.