Presumably there could still be some benefit from obtaining G.fast at these speeds?
The engineer said the socket was 364m of cabling from the cabinet. Where all of the cabling that supplies this socket goes is unclear - but if there was in excess of 50m of perhaps 'unnecessary' cabling, I would not be surprised considering how much is around the edge of the house and the fact that it seems to go through the roof.
G.Fast has a maximum range around 350m (on a good clean line) but this can be quite a bit less on some lines.
Your estimates place you on the very edge of G.Fast range.
You don't get estimates much lower than that.
Properties a little further away just get no estimates at all.
With you being on the edge of potential availability OpenReach allow the product to be ordered and during installation if it doesn't meet a minimum threshold then install will be cancelled.
As above I believe that minimum is 100Mb.
It sounds like the engineer wasn't able to obtain a connection (sync) at all, which is not surprising at 364m long.
Even with improvements in the internal wiring you may be out of range.
If you do manage to get a G.Fast connection near the bottom end of your estimates it is very likely that any small increase in downstream will be accompanied by a drop in upstream speed.
Your current 74/14 might end up around 100/7.
Is more Downstream worth almost halving upstream?
I'd throw a wild guess that you're just out of range.