it’s great to see so many good ideas flying around !
SFH > all your answers tend to prove that the problem is not “real”. There is no poor radio conditions, as TCH are fine, there is no SDCCH congestion, etc.
have you checked the qos per trx ? your answer was not clear about this. Ensure that the sdcch ts is located on a trx that handles a lot of tch traffic as well, then look at the qos stats for this specific trx only.
you should try to roll back to NH for few cells, see if it changes anything.