Further to the observations and comments from Chrysalis and kitz:
(assuming DLM gets reset when you restart a stopped line?)
Yes, would certainly assume this to be the case on a Huawei cabinet, from my own experience migrating from Plusnet to Zen on VDSL-2 service last December, DS SNRM started off at approx 6dB and then migrated back to 3dB by 1dB steps at 2 day intervals producing the associated DS synch speed increases.
During one of the DLM instigated steps my G.Inp re-tx profile also changed from low to high and I had to perform moderate modem DS speed clamping for a few days after achieving 3dB to encourage DLM to revert to DS G.Inp low profile again.
Provided you don't have excessive error rates there's no reason to think that progressive DLM action won't achieve DS 3dB SNRM.
As you are one of the very lucky few to have active Vectoring your error rates I would have thought will be minimal.
DS attenuation of 21.4db by normal standards would indicate that your line is long(ish), unless Vectoring effects this indication, however, should still be well capable of very reasonable speeds on 80/10 service especially considering the presence of active Vectoring.
From observation of the modem stats provided I would guess that your line is currently on DS G.Inp re-tx low profile, you can positively confirm this by looking at
Bearer 0 INPRein, 0 = low, 1 = high.
You are likely aware of the effect of G.Inp re-tx profile but just in case:
Low re-tx profile results in BRAS IP profile of approx 97% of DS synch speed.
High re-tx profile results in BRAS IP profile of approx 93% of DS synch speed.
ie., with low re-tx profile better data speed for any given DS synch speed.