An update. Thanks for all the suggestions. On a bit of a whim, I bought a Google Wi-Fi Mesh set up (three repeaters £170 ish) after reading that the modem bit of the router I have is alright - just the Wi-Fi that’s log.
One of them plugs into the modem/router then you position the others and the google home app helps you fine tune their location. This seems to have made a big difference (only one day in) with everything running smoothly. Even our TV that’s previously struggled as next door have their router the other side of the party wall to it. It meant setting up a new ssid/network name and transferring all the connected things - but that’s been fine (apart from the printer..).