Unfortunately I can't do that completely simply as the router is in roof space above the room where the master socket is, but I'll work something out. Also I'm not sure your diagnosis of the SNR margin is correct. Everywhere else seems to say the SNR margin needs to be at least 6dB, and higher is better. That makes sense to me: SNR stands for Signal to Noise Ratio, and the higher the number the more dominant the signal is over the line noise.

Different companies apply SNR differently. See this source about SNR and 6dB being the target: https://kitz.co.uk/adsl/DLM_profiles.htm So, the target on all lines for FTTC is 6dB.

I've been a Fritzbox user for the past two years, so can't remember exactly how the SNR is viewed on Drayteks. From what I remember from the time of my HR fault: if you increase SNR, you'll see more stability, but a reduction in speed, too. So, yes, more signal is good, but this comes at a price. Presumably, because you're so close the cabinet, it won't make much difference for you.

Also, your DSM will automatically (hopefully) adjust the SNR value: less noise and fewer disconnections = lower SNR. So, sometimes, you can run your connection at 3dB, but the caveat of that is that you're likely to have less stability.