We've traveled enough with my in-laws but I'm glad they backed out of the 2017 trip they were going to go with us on. They backed out more than a year in advance due to stamina concerns (and it's even worse now, they weren't even in their mid-60s back in 2017). We've done Cape Cod trips with them, we've done smaller visiting family trips, we've done many trips to the Lake of the Ozarks, we've done a road trip with them and they are about to go with us to Mexico (along with one of my sister-in-law's) to celebrate our 10 year anniversary.
However, there are trips that are not the best even as close we are in terms of vacation likes. Disney and Universal is honestly one of them. Years ago we went to Six Flags Over New England when we were visiting family in New England and that was okay but it was a one day trip. They've had season passes for Silver Dollar City for like 2 or 3 years in a row in the past but they don't go on the rides at all (and a season pass is very inexpensive). Even my sister-in-law (who is several years younger than me) backed out of that 2017 trip because in the end she didn't want the entire vacation to be theme park. She wanted to go to the beach for a day or two, she didn't want to do 5 days at WDW nor 3 days at USO (she's done a day or two at a time at Universal and that's all she would want and Disney maybe a day or two at most).
The usual advice is to start small with family trips so that you can get the feeling of how everyone does on it. After that though most people get to realize what types of trips are better for family member trips and what are just more stress than less stress. I do think WDW can work but it really takes making sure everyone is very clear about expectations and is okay with what that trip ends up being. I know one of our biggest concerns was with my mother-in-law who would not get out the door as quickly as we wanted so had they gone with us we would have been clear you guys just meet us at the parks, take this transportation (at that 2017 trip we were on site) and just meet us whenever. We would not have planned FP+ together because that would have been too stressful worrying about if they were going to make it and feel like doing it. A bit funny but our group of DISer friends we went to WDW/USO in May for one of them getting married in WDW and we all loved that we could finally have a trip with all of us there together. We loved it so much we planned a Silver Dollar City trip together (most of us in the group are local enough to SDC it's several hour drive) and the bride who had gotten married a couple months earlier remarked "I just want to say how great it is that we all got out the door on time, if this had been my family this would have never happened" (her family came to WDW for the wedding and they had some park days together).
In today's world with park reservations and
Genie+ (although the most recent update to be able to adjust and modify by person is very helpful) it's even harder if one or more people joining in have enough differences in touring. For the OP they have concerns over being able to keep up with them as well as planning for people who do not go into the evening at all. That may make for a frantic trip trying to work that in. It sounds like to me the OP's in-laws have enough of a difference in traveling patterns that this type of trip would be more likely to have friction.