You can get pretty close to guarantees.
Fresh git checkout on prod publish. Run all npm/node commands in ephemeral rootless containers. Only have publish token exposed and injected when you are actually publishing (not on install/build just prior). Separating users like you mentioned doesn't hurt bt doesn't sound like your lower-hanging fruit nor something that would likely save your bacon on its own without other workflow adjustments.
None of this is relatively difficult per se, just a bit extra friction which should be worth it to avoid these kinds of events.
Npm supports one time passwords, and iPhone has builtin support for the protocols that npm uses. Someone’s going to have to pickpocket me to hijack that repo. Hacking my machine with supply chain attacks would be much much easier.