In terms of futureproofing (if that’s possible) what should I be looking for in a new bike?

27.5″?

Presumably something like Hope hubs can be upgraded to cover most situations so it’s mainly about rim size and frame compatibility

I’m thinking about availability of decent tyres etc.

yes 27.5 has taken over the mantle from 26 after it briefly being held by 29. Don’t forget to check out boost hubs (wider) and forks/frames.

27.5 plus is the latest but I understand tyre choice is currently limited.

I’ve got a 5 year old 29er and am not worried about a lack of tyres, or forks, I think 29er will stay relevant.