Several years ago we took an extended family trip for my grandparents 75th and 80th birthdays, which included 13 of us at the time, the youngest being my niece who was around 5, I think. We've also taken a more recent trip, just before DW shut down for Covid actually, and on that trip we had our daughter who was 10 months old at the time. So, from across multiple trips I can touch on a few things.
Firstly, if you haven't already done so, definitely have a discussion amongst everyone about how much time you expect to spend together versus individual family units breaking off and doing their own thing. On our large, extended family trip we made it a point to have dinner together every night, but not much else. That isn't to say that we never went to a ride together but it was infrequent. My aunts and uncles are not big ride people and they found it more appealing to do some of the behind the scenes things and to wander through the World Showcase in Epcot. And my dad doesn't like crowds, so he would go out for a little while, do a few things, and then head back to the resort to relax, then do the same after dinner while some of the rest of us closed out the park (or tried). Thankfully this was all discussed in advance so nobody was ever surprised or disappointed by anyone's presence or lack thereof.
Must do things are always very subjective, but for my wife and I we have to hit the thrill rides, and on our last trip we had to see Pandora at AK and Galaxy's Edge at HS. We also wanted to spend time in Toy Story Land at HS, but unfortunately because of the way that fast passes were tiered we had to choose and I am a lifelong Star Wars fan, and with a 10 month old daughter we couldn't justify spending more than one day at HS because there are only two things she could ride there, Toy Story Midway Mania and the new Mickey & Minnie's Runaway Railway. Our daughter did love the classic rides throughout the parks, though, and was very interested in seeing the animals on the safari ride when at AK. One thing that we discovered as a must do going forward, at least for myself, DW, and DD, is a fireworks dessert party (if they're offered). The food was good, not the best but definitely enjoyable, but the experience and the great viewing spot for the fireworks made the cost worth it without question.
For the baby, we decided to buy her a seat on the plane and keep her in her car seat. This was an added cost when she could have been in our lap but it made everyone's flight more comfortable. DW and I could read when DD dozed or talk, get up without having to hand the baby off, etc. Plus it meant a row to ourselves, which was nice. We put the car seat in the window seat and didn't have to move her until we landed, thankfully. At the time DD was doing a combination of nursing and formula, and one thing that we loved was the individual serving packets of formula. We would throw 4 or so into the diaper bag and pre-fill her bottles with water in the room. If she got hungry in line for a ride, or even on one, it was incredibly easy to tear open the packet, pour it in the bottle, shake, and feed. They are a bit more expensive but they were so convenient that, again, we found it worth the cost. And speaking of the diaper bag, keep it small. The first day we used my normal travel backpack, which is not huge but bigger than the diaper bag. The problem was we started to use the space we had and by the end of the day we were both feeling the extra weight on our shoulders. The diaper bag forced us to pare down to the essentials and our backs thanked us.
Another must do after our most recent trip that could work for a family celebration is the California Grill brunch. It's only available on Sundays and it is expensive but the food was outstanding, and who doesn't enjoy bottomless mimosas? Plus you can go back for fireworks viewing on the rooftop viewing area (again, if they are taking place) by returning and showing your receipt. Not quite as expensive and definitely more hit or miss, for our extended family trip for my grandparents birthdays we had a great experience at Ohana. We had to wait a bit to be seated all together, but it's Disney so that's hardly unexpected. We got tables put together by the windows overlooking the lagoon and were there when the fireworks show took place. However, on a later trip DW and I returned to Ohana and while we didn't have a bad experience, had it been our first impression we probably wouldn't have gone a second time because there are just so many places to try. It just wasn't anything amazing the second time, but the first time was so good that we'll still give it another try on a future trip.
I'll stop here since this turned into a novel. Good luck with the trip!