score:5
Miri to Sg Tujuh is only 30 km, so that's an easy and cheap taxi ride (or Grab, the Uber equivalent in Malaysia).
Cross the border on foot. When bus services to run from Miri to BSB, passengers go through immigration on foot anyway, so this is nothing unusual.
Take a taxi (if you get lucky and one is waiting there) or use the Dart app (the Uber equivalent in Brunei) from Sg Tujuh to Kuala Belait, the nearest town with regular bus services to BSB.
Leave plenty of time due to customs and the transfers involved, and be ready to smile and ask nicely for help if there's a hiccup somewhere. I don't think it will be much trouble.