I had similar thoughts when i got mine, a bit of cx, a bit of gravel and a winter road bike. Apart from the turbo, the normal road bike does that as its got the nicest saddle.
The problem is those are kinda mutually exclusive. A winter road bike has slicks and full length guards, a cx/gravel bike has knoblies and doesnt. So by the time you have a spare wheelset and take guards on and off twice a week it becomes a real PITA. So you end up buying a second one……
My solution is to keep a spare (actually my summer lightweight) wheel for the turbo with the road bike and leave guards on that, and still ended up with two cx/gravel bikes (and a third mosntercross for a while).
Other good bikes to look for on ebay:
Specialized Crux and Trek Crockett – probably still too new to get under £500
Cannondale CaadX – possible if you shop arround long enough.
Planet-x XLA, similar geometry to the Full Monty but more CX than gravel (designed to be shouldered).
Canyon/Rose did decent but slightly race focussed models but rare as hens teeth 2nd hand.
Whatever you get, budget for new wheels, <£1000 Cx bikes have universally bad wheels. Even a set of Askiums or Shimano wheels with tubeless cx tyres will transform them.
Dont rule out rim brakes, in the dry they’re great. In the wet they’re still adequate. And because no one wants them they’re often sold for not much more than the groupset and often come with much nicer wheels!