Not sure if this helps, but when I’ve thought about going down to one bike before, I wondered if a way to do it would be having a rigid bike with three wheelsets:
A pair of 700c with fast rolling approx 40mm tyres, a rear 650b matched to a 29er front, with 2-2.3″ tyres or so, and a rear 26+ and front 29+ with 3″ tyres
Also a dropper (with an underseat lever) and a rigid layback post
May be Jones Loop bars or something
That way a quick wheel change and seatpost change would have you very different bikes: the 26+/29+ set up would be noticeably slacker and higher at the front than the 700c, and changing the seatpost would lengthen the bike if the dropper was inline and the rigid post laidback
And the 700c set up would have automatically slightly higher gearing than the 26+/29+ and 27/29.
Or you could have 2 chainsets set up with different chainrings
Obviously you could have one bike with various stems and bars, rigid and sus forks. But for me, I go on lots of quick 2 hours blasts and 20mins changing the cockpit around is simply not worth it