If you still keep snr on 8db I think not much more can be done. There is an issue somewhere. I barely get any errors with SRA disabled. I think is time to get BT to investigate.
Dito- reset DrayTeK modem back to how it was, try quiet line noise test again plugged into master socket only (modem unplugged) , if a low buzzing or squealing noise remains, time to call BT and explain a noise is on line so assume a fault, and mention that broadband speeds are low but reason for call is noisy telephone calls, hence they will be less likely to charge you if no fault is found.
I have seen a few BT vans at peoples properties recently, so hopefully availibility for BT Engineers is less restrictive 
Forgot to say, BT has/is bringing its call centres back home from India etc. so chances are a quality conversation with support operative is very likely 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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