hm your upstream is 38dBmv for power level which is a little low but not too far from 40dBmv which is a lower end of the recommended range for upstream power levels. I don't think your having a noise issue which would make your upstream channel change to only QAM16.
I think your problem is an over utilized upstream channel (overcrowded) I think channel bonding would help possibly if your node is not maxing out on all of the channels. My area is bonding 4 upstream channels right now. I would assume that upstream bonding will come to your area sometime this year or next.
This has gone on for a week. I am paying for 30/5 and getting 30/2. The techs Arris Gold modem did 50/5, but my sb6141 and two other DOCSIS 3 modems will not get me 30/5 - I'm not a techie, and tier 3 tells me I am "the only one with this problem", at last contact ( yesterday ) I was told this was being bumped to engineering. All I know is I'm not getting what I'm paying for - and I own the modem and do not lease it.