So, I have had a cx bike for ages and it got used as a road bike which goes down a few trails. It's been commandeered by my middle boy and since hes riding it 3 days a week I'm not going to take it back from him.

I have a road bike, which I dont like (uncomfy and not quite my size).

So, new bike on the horizon of the gravel / cx / b roader / not a road or a mtb bike or whatever they're called

The choices are either get a new frame and fork and put the kit off the road bike on it

Prime frame candidates are
Nordest Albarda
Brother Mehteh

Or alternatively I could buy a full bike, such as an Akrose and put the best bits from the road bike onto it.

This would mean I have another bike to ride / put on the turbo when needed but its also another bike taking up room.

Either way I'll end up with a nice bike I'm sure. But which direction to choose...

Depends what spec the road bike is, how worn it is and how much will transfer to the new frame and forks? I'd probably sell it whole.