I will try @egc's suggestion. My actual Starlink IPv4 address seems to typically end up being something like 100.67.x.x despite the 100.64.0.1 gateway, so it seems like a reasonably safe bet that the exact address won't collide.
I'm still considering just abandoning Mullvad's filtering and using either Adblock or Adblock-Fast. Any comments comparing the two (differences, pros and cons) would be very welcome.