That's tough. I totally understand your thinking in wanting everyone together for the first part of the day to guarantee a little fun together. It also makes sense to do the 3 pre-scheduled FPs somewhat early in the day that way you can try to get more afterwards on the app. With that line of thinking, the 3 FPs would then be for things the entire party will do.
The downside to that, like you mentioned, is that if some people won't do the attractions with the longer waits, then the FPs will be used for things with shorter waits and those wanting to ride the coasters and such will likely be stuck in a long standby wait later.
When I travelled with family who wouldn't ride the "big" stuff, I planned FPs for the rides with the longest waits for those who would ride, along with a smaller attraction nearby for those who wouldn't. Like some would have FP for SDMT, others for Pooh. Some for Space, others for Speedway or Buzz. That meant we were still able to walk/talk together between rides/get a snack. We did those pre-planned FPs first, because it makes sense to schedule them early so you can get more later, but also because the kids got to do some of the things they were looking forward to early in the day without having to wait too long since they had FP. It avoided the constant "can we ride x yet" later in the day when we were doing smaller rides together since they were able to do those rides first thing and it seemed like the kids were better able to slow down and enjoy the family time on the smaller rides. We didn't do rope drop though and got to the parks around 11 most days.
If you guys are planning on hitting rope drop with the whole group, it would make sense to have everyone split and take advantage of the shorter waits for the big rides for those who wish to partake. If you do something like that, some of the bigger rides could be taken care of with little wait and the FPs could be saved for things to ride together. Maybe have the first 1.5 hrs (not sure how long until the lines start building up) be split with part doing roller coasters and the others meandering around, grabbing a snack, or hitting up a smaller ride or attraction. Then, meet up and do your 3 FPs together on smaller rides/attractions, then take your midday break. Or, have the thrill seekers go alone (assuming there's at least one adult of course) to the parks to take advantage of rope drop while the rest enjoy a quiet morning at the resort, then thrill seekers come back to the resort for lunch and you all go the parks together afterwards and do your FPs together.
If you're not planning on hitting rope drop, maybe have everyone split for FPs right after you get to the park, then after that, take your midday break. When you return to the park, spend a few hours riding smaller things together either standby or with FP (which you might be able to pick up some of the smaller rides as 4th, 5th, etc FPs later in the day). Even if you do have to ride them standby, (depending on the ride, of course) many of the smaller rides don't have as long of waits and you'd still get to spend that time together in the line.
Another way you could work it is pick 1 or 2 of the FPs as something everyone will enjoy and the other 1 or 2 as something that you split up on.
What I would do (if everyone is at least somewhat familiar with the parks) is have everyone select their top 1 or 2 things they want to do for each park, then go from there, trying to see what makes the most sense with planning FPs. For instance with Epcot, if half the group really wants to ride Soarin, and most would really like to ride FEA, it might make sense to have part of the group rope drop Soarin and make a FP for everyone for FEA later on since they both have longer lines and you can only FP one of them. For AK, if half want FOP but half would love Navi, split that FP up with half doing each. For MK, Peter Pan has notoriously long lines so if that's something that's a priority, schedule that for everyone.
Keep in mind that you don't have to follow the same general game plan each day. If one day you go the parks at 11, ride FPs separately then meet up later, it doesn't mean the next day you can't hit rope drop and have the thrill seekers ride the big stuff then meet up and do your FPs all together. Then the next day, spend the first few hours riding FP stuff together before splitting up. Just try to see what makes the most sense for each park, taking into consideration the top picks for each person. And if you have more than one day at any park, that makes it even easier since you'll have more pre-scheduled FPs to work with.
I'd also suggest typing up an overview of each day with a general outline of what park, when you plan to leave in the morning, scheduled FPs, ADRs, and a rough idea of when you'd like to be together and pass it out to all of the adults so they know ahead of time what's going on. I'd suggest getting input before your 30 day mark and if you have a rough idea of the plan, passing it out before you schedule the FPs that way you can get feedback from everyone and make changes if necessary. Then bring extra copies on the trip in case someone forgot to bring theirs, that way there's no confusion and things run smoothly with everyone understanding and hopefully being happy with the plans. I've found people get less grumpy if they know ahead of time that their favorite ride IS scheduled for later or (for the non thrill seekers) that we WILL be riding several things together later, we're just splitting up to do this first. Oftentimes once you're in the parks with a larger group, it's hard to relay verbal messages and have everyone hear it while you're walking so you end up having to say 8 times where you're going next and it leaves everyone frustrated.
