The OpenReach FTTC DLM is different to the BT Wholesale ADSL DLM.

Every line on ECI cabinets has a fixed 6dB target SNRM.

Every line on Huawei cabinets starts at a 6dB SNRM target.
If a line is stable enough the DLM can lower it to 5dB, 4dB or 3dB.

Unlike ADSL there is no 9dB or 12dB on the OpenReach DLM for FTTC.

Upstream target SNRM is always 6dB.

Important to note that the target SNRM and your current SNRM are not the same thing.
A modem should try sync at the target SNRM, but the current SNRM can move up or down after sync.

My understanding is if the router is syncing at 80000 Kbps and 19999 Kbps with a 6dB noise margin profile then DLM will not need to lower it down to 3dB as it is happily syncing at top speeds at 6dB.

Correct.
Your line is only just syncing at 80Mb.
It's likely as more take up FTTC on your cabinet that crosstalk will reduce your sync.
At this point DLM may reduce the target SNRM and get you right back to 80Mb.

Upstream target SNRM is always 6dB.

But in my case the Upstream SNRM is 15.40dB
https://i.imgur.com/0oZtZHE.jpg
It has been that way from the very beginning
Downstream Upstream
Actual Rate [Kbps] 80000 19999
Maximum Rate [Kbps] 80534 19999
Noise Margin [dB] 6.30 15.40

6dB should be the target, but I am guessing since my router is syncing at the max 19999 upload at 15.40dB that too should happily stay that way unless crosstalk comes to lower the speed and DLM tries to drop it down.

The only tiny fluctuation I see is for download noise margin. On rare occasions it would hover between 6.30 to 6.40dB. While Upstream has been rock solid at 15.40dB.

This points to some very stable noise margins. With ADSL the noise margin would constantly fluctuate at every refresh of my router stat page.

My line Attenuation was 25.5dB with ADSL and with FTTC it's down to 17.90dB.