I've done some further testing and I think I've narrowed it down to the "Unlocking em(4) a bit further"-patch [0]. With the patch reverted, I haven't seen any watchdog timeouts yet. I'm currently running the router with the patch reverted to make sure the timeouts don't happen again.
[0]: https://www.marc.info/?l=openbsd-tech&m=144347723907388&w=4 -- Gregor
