sounds like you are getting somewhere 
the DBDC has always had these problems
I'm hoping some day someone with the skills with MT76
will sort is out one day but yes the DIR-853's had the same problems
the DBDC radio may be limited to non DFS channels
but the 4x radio should be fine like my 1960 & 878
as the 2660 is the same it;s fine for DFS as well
so i don't understand that
my only recommendation is to make you code available for other to better
and if motivated add it to openwrt via a pull request