Hi Michael,
You've come to the place. OF COURSE we'll suggest two bikes as the inevitable solution. More = better, N+1, etc.
Meanwhile, there are solutions to swapping, for instance
I think that site is still in business, but I'm not sure. He was after swapping from flat to drop bars, though.
I had a page open recently for a builder that incorporates S&S couplers; if you search for those and 'disconnects', you should find what you're looking for.
For an all-round bike, the Sam would be my choice, too. Heavy enough for anything that's not crazy, lively enough for road riding, cantilever brakes for tire flexibility. You have to get the top tube length right. It would help if you already had a bike or three where you can try out different bars. I'm 6 ft even, 88 PBH and I like a 57-58 cm top tube. Even then I need a short stem for road bars. I use Noodles, with moderate reach (95 mm I think). I have one bike set up with a Northroads bar, it's got a 59 cm top tube, and I'm using the longest stem from the bin at 120 mm. It's an old Rockhopper and i've never run a drop on it.. probaby be on a 60 mm stem, and have to push the saddle forward, which is not ideal. Get a cheap older road bike, a stem adapter, and different 'threadless' stems for painless playing around.
I know that Rivendell pushes the idea that you can put different cockpits on the same bike, and they're not wrong...it's hard to make a bad bike.. but I think we're better served by figuring out our fit and what setup we want, and then buy the frame to match. I could ride a 57 or 60 Sam with swept-back bars, and I'd probably buy a 54 to use with drops, based on the effective top tube. I'd be unhappy if it looked goofy with the stem at saddle height, which it might.
FIgure out which bars you like, and take a look a their dimensions. Switching from a drop to a flat bar makes a lot of sense, but swept-back, especially the long Boscos and the like, can put you bolt upright. Some people like that; it feels awkward to me.
I keep re-using this picture, since it's pertinent to your question.. which comes up frequently.
Personally, I'd want two bikes, so one is always ready to go.
Put your money towards the riding you love the best, get a beater for the other kind.
cheers -mathias