What do you reckon, is this something I should…
Well, I ended up switching the battery. Upon closer inspection, the semi-old one didn’t look too great, so just to be on the safe side, after removing the battery I cleaned the whole area with baking soda paste to neutralize any remains of the nasty stuff there might be and installed a battery holder and a lithium battery (+removed the charging stuff from the board).
The IC right next to the battery had a couple of legs that were not as shiny as the rest, so if anything should fail in the future that would probably be where I’d look first, but the whole rest of the chip and the traces underneath it all looked nice and clean, so I decided to just leave it alone.
Cleaned a few keyboard contacts and a scratchy pot while I was at it, looks super clean now, everything works perfectly and Tubbutec ModyPoly is currently on it’s way (going to get Polysex later as well). Sorry for the off-topic bits.