I suspect, it was a RAM problem, but not in the way I expected. I play Underrail on a mac, through a wine port, and it was due to the nature of wine. For some reason wine doesn't allocate enough RAM for the app to run smoothly. It led to occasional lags, but nothing serious until these maps. I suspect that the RAM limitation only became a crash-kinda problem when on these big maps.
(Resets were tried, and RAM was supposedly flushed, so that didn't exactly help.)
I got an old PC reinstalled with way worse specs, transferred my saves, and finished the game. Without any issues.
So yeah. Thanks anyway for checking back.