Sport touring... right up my alley.
First you can absolutely tour on a 'Busa... I did it for years.
It's a fantastic bike for it, if your body is still reasonably flexible. No good for two-up though.
I did the Ducati thing. It sucked. Too much maintenance, and handled like it had a hinge in the middle. E36 M3 brakes. Way more bike for the $$ from Japan. Also to cramped for serious two-up distance. That said, there is no better sound on two wheels.
IMHO, the R1100RT/R1150RT is an excellent choice. I had an R1100RS and it was fantastic, but would have been tight two-up. Great handling, reasonable power, and all-day (like 1k miles a day) comfortable.
The earlier bikes had an odd size rear tire that was kind of hard to find on the road, but the later bikes may have gone to a 180/55... not sure. All of these are getting long in the tooth, and parts can be expensive. Air/oil cooled bikes can get hot in traffic in summer. From ~2004(??), when the BMWs went to 1200s, they got exponentially more complex and even more expensive to keep running.
Gold standard in the 'older' bikes is the ST1100. I had a couple over the years, and they are really good bikes. Smooth, comfortable, and deceptively fast. Honda reliability. Unfortunately, a little Honda personality, too... or lack thereof. Kind of like a two wheel Accord. Big enough to be great for two-up.
The ST1300 is faster, roomier, heavier, and by all reports pretty boring. Even more like a two wheel Accord. I've only test ridden them so I can't comment much more than that. They do have really good brakes.
FJR, IMHO is the gold standard in newer bikes. My BIL has had several of them and they're great bikes. He keeps buying one a little newer than his last as the prices drop. I don't have a lot of time on them, but I was impressed. The earlier 5 speed bikes can be a little buzzy on the highway. I don't know what prices are like on these right now, but for what you said you wanted, an FJR would probably be my choice.
Another option is 'adventure bikes'. I had a VStrom 1000, and it was probably #2 on my 'all-around' best bikes list, and I've owned a lot of bikes. I rode it coast to coast, and border to border. I rode it on the track. I rode it 1350 miles in 22 hrs.
Fantastic motor. Great Ducati like noises. Suspension good enough to grind pegs. Roomy and comfortable. The earlier bikes don't have a big enough alternator to run heated gear for rider + passenger. It also didn't get the greatest mileage, and hard riding loaded up could get the fuel light on at ~120 miles. Wind management/weather protection is not as good as sport-touring bikes, but that's true of all of the adventure bikes.
IMHO, stay away from the smaller version (Weestrom/ DL650) for two-up, loaded up touring, especially if any altitude is involved. You'll feel like you're back on the Grom.
Anything based on a sport bike will be too small for two-up touring. You can do it, but you won't like it.
I owned a 1500 Gold Wing for a while, and have spent time on the 1800s. Nice and roomy for two up, but the feet forward, sit up straight position was not compatible with my back. Serious lack of ground clearance for the twisty bits. Heavy... there's a reason they're called Wingebagos...