Dealing with those in turn:
a) any certainty that the delay date were ever to materialise (albeit some 3 months away)
No certainty. VM spend tens of millions of pounds each year increasing capacity, but you wouldn't know that from this forum, and that's largely because VM's modus operandi is to over promise and under deliver. Behind the scenes, the "fix date" for an over-utilisation fault isn't that, it is a "review date", meaning that in some/many instances the company has a date to offer customers, but no work is done, and they hope that the problem goes away. And by problem, I suspect they mean unhappy customers. In the majority of over-utilisation instances reported here, there are multiple missed dates, and as each new one becomes due, the date gets pushed back (one of many examples). Forum staff can't see behind the scenes to view any prior history of made up dates, nor can they see if there's any real plan to fix things, and they are unfortunately pig-in-the-middle between frustrated customers and the forces of corporate idiocy who have this policy of made up dates.
b) any confidence that when/if it does get stated as completed this would be noticeable from my end
If VM actually chose to invest the considerable resources needed to address an over-utilisation problem, yes, you would see a night and day difference. The company know full well what causes over-utilisation, and they know full well how to resolve it, and sometimes they even do that. Trouble is that there's a longer list of capacity issues than there is either budget or skills to fix (and in some areas, as soon as its fixed, they simply sell more contracts and repeat the problem). No matter how firmly VM assurance you of an imminent fix, the evidence is that those promises cannot be relied on as per link above.
c) the option to buy additional equipment (not sure what?) and switching the Hub3 into modem mode may resolve the issue without a) or b); or
No point, a better router or mesh system solves wireless problems, won't make a blind bit of difference to a capacity constraint on the local coax network, I'm afraid.
d) change ISP provider as you suggest - any recommendation?
I hold Zen Internet in high regard, but AAISP, Aquiss, CIX, IDNet and uno all have excellent customer ratings. Avoid the big ISPs, they all use Openreach, but there's a huge gulf between different companies ability to get the best from Openreach. Light reading on the topic. If you're still in a fixed term contract you may have to have some firm discussions with VM to force them to release you without penalty. Happy to advise on that if needed.
