2 already has the empty frontage roads carrying buses. There isn't a need for separate bus lanes.Yeah Rt2 from 128 to alewife is always going to be way too big because it was built at 8 lines with the expectation that it would tie into the inner belt etc.
So two of those four lanes could easily be repurposed as bus lanes. Throw a couple park and rides in between there and 128 and you have a good red-feeding BRT system. Could also be extended up 128 from Lexington to Burlington with a new BRT ROW along the powerlines. Then build a huge garage at the end of rt 3 and declare victory.