Is it likely that the modem is clocked in at 6.5Mbps but the real throughput is always stepped down to 2Mbps (I think this is unlikely based on other plots from friends)
Your stats confirm the modem synced around 6.5Mb.
The upload "IP profile" on the line as applied in the cabinet is showing as 10Mbps after the BTW "advanced diagnostics" test, but is it possible that this has got stuck at 2Mbps?
The downstream IP profile is calculated from the actual sync speed.
The upstream IP profile is always the package limit. They don't limit the upstream by IP profile like the downstream.
Is is possible that the stepped reduction I'm actually seeing is the cabinet trying to stabilise upload rates, and is therefore healthy behaviour?
No. The DSLAM does nothing like that.
The DLM controls things like that, telling the DSLAM what profile to use.
The DLM isn't limiting the upstream.
It sounds like an issue beyond the DSLAM, which is down to BT Wholesale.
Ask the engineer to run a speed test (if 1 shows up).
Try stressing to BT that the line and sync rate is fine, but the throughput is stuck.
There's very little an OpenReach engineer can do for some throughput issues. They are responsible for the line between the home and the DSLAM, and the fibre between the DSLAM and the exchange.
The issue appears to me to be speed to the wider internet from the exchange onwards, which is where BT Wholesale take over.
It may need a circuit rebuild. Basically BT Wholesale build the line from scratch.
I'm confident it's a BT Wholesale issue though.