I have long standing knee problems recently exacerbated by Thai Airlines flight (work trip couldn't choose) this past week Bangkok to Hanoi which used stairs and bus to get to/from plane for Bangkok legs. Hanoi legs used sky bridges. I am going to Hong Kong next week and trying to decide on airline. Does anyone know if Emirates or Cathay Pacific almost always use sky bridges departing from Bangkok?

You'll be fine on either of those.

 

Usually the bus issue is because an aircraft is being swapped from a domestic route to an international one and therefore can't be moved between airbridges in domestic and international concourses - so they plonk it at a remote gate and bus you out.  Should never happen with Emirates or Cathay.