I have biked from Zurich to interlaken-Gstaad on almost a similar route last year.The second leg is definitely great, one passes over several lakes.

Brunnig pass is not a real pass, I mean its pretty easy, u dont feel u climbed a pass, also because its all in the forest.......just before u hit the pass, there is a 13% section on the biking route for over a Km to reach to the lungerer see, that bit is hard . But the only 2-3 km of what you will climb of the real brunig pass has a lot of traffic ...I think there is one off road alternative which is more steep and unsteady.

coming down from the Brünig pass is is a much longer way and I will stick to the main roads here, because the national route really takes one through steep ofroad descents which killed my brake pads in wet weather.

hope that bit helps...best of luck and I envy your riding plans but cant join this time around..

need to dust me bike now