It should be do-able. As would a wireless mesh system.
The trouble is the cable/powerline bridge/wireless bridge would have to be dedicated to just that role, as that connection is technically still outside of your network and is part of the connection to the outside world. That is, it’s insecure etc.
There are people on here that know more about wireless mesh and powerline than me. I’m an old-fashioned techie. Real networks have wires. (I upset someone the last time I said that, so let’s put a big LOL on the end of that…I’m not being serious, people, so put the tar and pitch forks away).
If you want to remain on FTTC for the short to medium term ( even though FTTP is available to you ) that can be done , with the proviso’s, if you change provider or renew with a new minimum term with your current FTTC provider, the new provider or contract renewal with the existing ISP may trigger an unavoidable upgrade you to FTTP , the way that is avoided is by staying with your current provider but on the deal you are on, that could mean being on an out of contract basis and paying a premium for being outside a minimum term,….it’s also likely in the future ( but probably a good few years away ) FTTC will be withdrawn and anyone who has FTTP available but refuses OR FTTP will simply be told that they will lose the FTTC service and to look at alternative providers.
existing openreach ist underground (front of house), new fttp comes through poles in the backyard.