I don't have any specific advice for you OP (I book offsite and get a two bedroom/two bath condos offsite on most trips that makes this a little easier for us in squeezing in an extra person) , but I just want to say that I feel your pain. Last September I planned a Estes Park/Rocky Mountain National Park trip with an adult group who would commit (immediate family) and a live-in boyfriend (they have their own apartment) and boyfriend just decides last minute, never more than a month ahead of time except for this next trip (I just got a commitment two and a 1/2 months ahead of time, and I think I'll faint.). And for 2022, I am doing two more of these, one to Orlando in March and one to Southwestern, CO in June. The boyfriend loves to travel (especially if we are paying lol), but has never been a planner and is always thinking, "I might get that better new job that I can't take off from, etc." and typically will never commit to anything more than a month out.
Most of my Southwest frequent flyer points go for tickets for the boyfriend, as I can book way ahead of time to use less points, but then cancel last minute and get all my points back. In our March trip, I have five who have committed for the whole time way in advance (actually the boyfriend as I mentioned above did just commit last week), but what I did 11 months ahead of time was book a two bedroom / two bath condo at the Sheraton Vistana Resort that could sleep up to eight (but not really) - king bedroom, second bedroom with two doubles (a bit tight for two adults in a double - really prefer one adult per bed here), and pull out bed in living quarters. That's an easy setup to throw an extra adult into if need be, as long as some are reasonably flexible in sleeping accommodations/privacy. We are doing couples in the master and pull out bed and adult singles on the doubles in the second bedroom. The two full bathrooms and extra space are super nice, but we're just doing two Disney days too. Offsite condos are pretty inexpensive for extra space. Onsite Disney not so much.
On tickets, I bought the two day Disney tickets for the boyfriend ahead of time, even though I didn't know for sure he was going. In case there were capacity restrictions which pandemic stuff and because it was a busy spring break time, I wanted to make sure we could all get in. I had a plan B though too. My favorite nephew lives in Orlando and I could always twist my pinky (oh ouch) and do an extra solo Orlando trip to see him, change those tickets to my name and go to the parks on my own or with my favorite nephew. Seeing family down there and going to the parks is that much fun for me. I'm always up for an extra Orlando trip.
For our Southwestern, CO, trip in June, I have firm commitment for three immediate adult family members, and again it's a maybe for boyfriend (no commitment for him on this one). I booked boyfriend's flight with my Southwest points. He can decide as late as 10 minutes before the flight and I can still get my points back. I paid cash for the other air. For two nights I have two fully refundable hotel rooms (Hampton Inn). I will cancel one if boyfriend doesn't come and the three of us will stay in one room instead of two to save some dollars. DH is fine with immediate family in one room and one bathroom, but with the boyfriend there DH want his own bedroom and bath. Then and I am paying for more than I need if boyfriend doesn't come, I booked a two bedroom / two bath condo for five nights. It's hard to get something where I am going that is nice last minute, so in this case I will most likely keep the condo even if boyfriend does not come (costs me a bit more, but should be super nice and we will enjoy the extra space even if it's just three of us). Kid's boyfriends can be expensive lol. At least we really like him.