Max. With all due respect I think it's time you faced a few facts.
You are already fortunate enough to have one of the best FTTC connections in the UK and I cannot comprehend what you actually need the additional bandwidth for.
Over the past year alone I have seen you place orders with numerous other ISP's only for you to cancel them within a few days. This is unfair on them because it places additional burden on them from the numerous questions that you also obviously bombard them with.
Re Plusnet. Whilst there is no excuse for the billing issue - I do feel to be fair that their support have been admirable when it comes to the amount of support tickets. Also bear in mind that I'm not aware of any other ISP who is quite so willing to run so many GEA tests and many will refuse to do so unless their is an active fault on the line... and even then some of them won't share the results with you. I also feel that you should not be too harsh because some of the deals they have supposedly offered you are better than what I've been able to negotiate. If you don't have an active fault on the line then I do not see the point in going else where when it is quite obvious that funds are limited and you have other outstanding debts.
AAISP is a fantastic ISP with some of the best support agents anywhere.. but also be aware that they are very technical and some of them can be very frank. RevK for example does not suffer fools gladly and he certainly is not best known for his tolerance towards people who he feels are wasting his time.
I hate to sound pessimistic, but I do not feel that g.fast would be your answer. Based on my daughter's experience g.fast is highly unpredictable speeds are never the same and they have even had a period where the DLM got a bit too aggressive and their speeds were worse than on VDSL2. I think their line length is similar to yours and if it was up to my daughter alone, then she would go back to normal FTTC as soon as their contract has expired. My daughter is not one to bother about speeds nor DLM or latency, but if she is getting fed up of noticeable changes then I really do worry you would cope. I honestly fear that the constant changes in sync speed could affect your mental health.
I have seen you say at least half a dozen times within about the past 6 months that you agree g.fast may not be the best thing for your circumstances. Yet within a week you forget all the previous advise and information given to you by various members of the broadband community (not just here but on numerous sites) and it's back to square one and yet again you are placing more new orders and around the same circle we go again.
-----
I kept out of this latest discussion because I've felt I have already explained to you at least 3 or 4 times in the past why g.fast speeds are different for premises in the same street. I even went to considerable length to type out a reply showing the layout in my daughters road with the location of the DPs and JBs.. yet here we are over a year later and you are still asking the same questions. I'm not sure what you are hoping to achieve, its almost like you will ignore the theoretical facts if they are not what you want to hear.
This post may sound harsh.. but honestly Max for the sake of your health and finances you really do need to take a step back and consider your options carefully before you start going down a different path which could just end up being the road to disaster. I really am concerned that your constant chasing of need for speed and perfection is beginning to spiral out of control.