I was the lead planner for our group of 11 this past spring. Like you, I'd done the planning for a past large-group trip and it went super well, and so I felt a lot of pressure and stress going into the next one, particularly with the changes surrounding Genie+, park hopping, and park reservations.
My best advice is to just relax about it. I know how hard that can be, I am totally Type A when it comes to these things, and I totally understand the pressure you're feeling under the circumstances. Everyone is spending a lot of money, and you feel like it's your responsibility to make sure all goes well. Trust me, IT'S NOT ON YOU. You have a sick kiddo, that is obviously Priority #1. All the other people going on this trip have access to the same resources you do - they can google, research, and plan too, if they're so inclined. IF they are not helping, they cannot complain.
As far as actual tips for making this work for your group, and for you and your mental health, my best advice is this. The whole group does not need to stick together the whole time - not only do they not need to, but it mostly likely won't even be possible if you're using Genie+. Fastpass+ worked SO much better for large groups - Genie+ makes it harder to get back on the same schedule once you split up. Not impossible, but harder. So just go in expecting and being flexible, and you will be fine.
One thing that worked for us is that we are, in general, a group who likes a good afternoon break. So while we may have split up at some point in the morning, if we wanted to spend the evening together, I'd use the afternoon break to book rides for the entire group in those two hour increments. By the time we'd go back to the park after dinner, I'd typically have 3-4 Genie+ experiences all lined up for the group and we could just go from ride to ride.
Also, make everyone responsible for buying their own Genie+ daily BEFORE 7am. We were all linked in
MDE, but my rule was that if your Genie+ wasn't bought by the time I went to make my first Genie+ for the day, you're on your own. I was not purchasing Genie+ for a group of 11 and then collecting money for it afterward.
As far as ILL's, talk with your group ahead of time about who wants to do what. That way you know going in how many/who to book for when those ILL days come around. Double check the night before and just get a final headcount.
I did use TouringPlans to plan which days for which parks, and to make our room request, but I didn't even attempt to do a true touring plan with it. With such a large party and the unpredictability of Genie+, it just seemed like a waste of time. Instead, I used an excel spreadsheet to get an idea of which rides and attractions were priority for the group on which days.