Yep, that's fixed it!
@stephen tinkering with a new (to me) laptop/tablet hybrid device recently has reminded me of what its like to (frequently!) get burned by Linux OOfM.. especially given how demanding Nix can be, even more so with ZFS in the equation!
Whilst tinkering and looking at how other people were handling it, I stumbled over this - https://github.com/rfjakob/earlyoom
Haven't tried it yet, but it felt promising enough to add it to "the list"!
It even has a #NixOS module 