Tusker House at AK is a good buffet and a fun meal. Enough there to satisfy picky eaters, but some fun ethnic options as well for adults who would like to try something a little different. Also, since there is a Rainforest Cafe in DS, you could still do that as well a different day.
At HS, 50s Prime Time might be a better option for a large group than Sci Fi because of the awkward table configuration at Sci Fi. We actually did sit all 9 together at 50s, so they do have some bigger tables. At Sci Fi, you may end up sitting with your back to your group, unless you sit at the picnic tables, and if you can't sit in the cars, what's the point. There is also a buffet place at HS, not sure it's anything special, but buffets are safe options with so many.
Crystal Palace in MK is a fun buffet with WTP characters, but the food is nothing special.
We also really enjoy the Thanksgiving meal at Liberty Tree Tavern. I believe dinner is still family style all you can eat. Might be another good option for a big group. We find when we are all together, buffets and family style meals work best for us.
Maybe try a fireworks dessert party as a meet up one night instead of dinner. That could be a good option that wouldn't interfere with nighttime entertainment, as it would be the entertainment.
Whispering Canyon at WL is supposed to be a fun meal, lots of craziness going on during dinner. Plus you could use the time to explore the hotel and nearby FW together. Maybe all watch the Water Parade together, see the gyser, go to one of the campfire s'more sing alongs, or take a carriage or horse ride. Those would all be good multi age large groups activities. If you all wanted to commit to spending one evening together. Maybe late lunch, early dinner there, than head over to the fort. You wouldn't need any park tickets for any of that, so everyone could do their own thing during the day.
Maybe do the Luau or Hoedown altogether as a group or Ohana.
Sorry, I'm drawing a blank at Epcot. I just realized, we never really do sit down meals here. The Aquarium restaurant could be fun for all ages, not sure how large group friendly it is though. We have a local one, so I've never been inside the one at Epcot. We try not to do things we can do at home on vacation.
Two other things I just thought of
1. Prepare to wait longer than usual to be seated and for meals where you order to take longer than usual. 50s Prime Time ended up being a 4 hr meal for us. I had an ADR for 12 something, and we didn't get finished until after 4:00. 2 hour wait to be seated because of our our party size. Then, it took an hour to get our food. Then, an additional wait for dessert. I can't imagine how long it would have taken if we had of been slow eaters. This wasn't the norm, but it can happen. Most of our meals we waited 15 to 30 minutes to be seated, maybe a few times 45 minutes, and most of our meals took around 2 to 2.5 hours total, waiting plus eating included. The point is don't think you'll be done in an hour or less, even at a buffet.
2. Meeting up in DS to go bowling one night might be a good family activity, or going to the dine in movie theater. Both would be large group friendly.