Just a bit of an update here.
Since the DLM resync to 3dB, my line had been stable - no resycns, no drops just lovely fast speeds.
However, on Thursday (22nd) evening at 22:45, my line resynced due to Remote Defect/DLM. Looking at my stats around that time, since around 19:30 my Bitswaps/min on the downstream went to 0 and remained there til the resync. Consequently, this caused G-Retransmit Corrections to go from their usual low level to over 100,000 a minute.
I'm not quite sure why Bitswaps/min dropped to 0 like that, but clearly DLM was doing its job by resyncing the line and correcting the issue. After the resync, I gained a tiny bit extra on the downstream and now sync at the full 79999/19999. Interestingly, the modem's measured attainable sync post-resync increased by 1.5mbit on the downstream, and decreased by 1.5mbit on the upstream.