Your estimate of the distance to the cabinet is way off , the speed you get is appropriate for a distance around 2500 m to 3500 m from the cabinet (not half a mile or around 800 m ) , in the past , at this distance from the cabinet, exchange based ADSL would perform better than FTTC , but that’s no longer on offer especially as you have a telephone service associated with your broadband service , if you were moved from ADSL to FTTC (because of the PSTN switch off ) and your problems coincided with this , then it may be that your FTTC is effectively working at its limits , that unfortunately is just physics, there is a point where the service just can’t work reliably.
To give any real help ( using real data ) you really need to post your router connection statistics, preferably twice , once when performance is poor , then again just after you have reset the router and performance has improved, without this then this thread is somewhat pointless , you have a service you are disappointed in , the provider has offered you the option to leave penalty free , but you presumably are not going to take that option, your provider has (on the face of it ) explored every avenue to improve service , all replaceable items have been replaced , your home checked (even though arguably this isn’t your provider responsibility) and Openreach have visited multiple times .
If your assertion that the problem is it’s the failing Openreach copper network that is responsible and OR are unwilling to replace or repair it as it’s uneconomical with FTTP on the horizon then what exactly do you expect, you have in effect been given a choice , accept this services limitations or make alternative arrangements, and if you do have an alternative from a low earth satellite service for example, then perhaps you should consider that .
FYI , if you do post your router stats , could you try and make sure the images are sufficiently detailed (from a resolution perspective) as your DSL (BTw) return is impossible to make out (for me at least )
