Future-proofing home network cabling in a building with poor WiFi performance
IT industry is very keen on getting things to work on UTP cabling because it is cheap, robust and has a huge installed base. So the answer is CAT6 or better.

Fiber cabling on the other hand is expensive, connectors are sensitive to dust and without careful handling it will break sooner or later.

20 years in IT is an extremely long time, 20 years ago we had totally different types of networking technology (Token Ring, Decnet, ...) and Ethernet was based on thick or thin coax. Your best bet is to install empty pipes in which you can run new cables whenever you require them. 'Loze leiding' that is what we call it in Dutch, don't know the proper English word for it.