First of all, I think it is very special that you are taking this trip with your 75 yr old aunt! My mother had her first and only visit to WDW at 89, and she loved it! At 92, she got to visit
Disneyland. She had great fun driving an
ECV in WDW (first time for one of those, too!) but was more comfortable in a wheelchair for the DL trip.
Late April is generally very warm. I hope you luck out and humidity is not a factor. The heat is quite tolerable without the oppressive humidity.
Your schedule is ambitious, so really stick to your guns about taking your time. It only take one day of pushing yourselves too hard to ruin the remainder of your trip. It would be nice to be able to alternate park days with resort days. But, with no built-in recovery time, make use of the months ahead to see if there are any must-do experiences for either of you. Don’t make it a long list, but doing one or two priorities each day can be very satisfying. Different things resonate with different people, and your aunt may be harbouring a deep wish to ride the Jungle Cruise above all since watching Walt talk about it in her childhood. I confess, I wanted to ride Dumbo and Peter Pan more than anything! I was 40 at the time.
One other piece of advice I can offer is to find a way to inject some Disney magic into these months before your trip. Anticipation is something to be savoured and it is best when shared. If logistics permit, get together with your aunt from time to time. Watch a classic or new Disney movie, with some Disney-fied snacks. Get together for a meal and try a Disney recipe from a restaurant you’ve enjoyed or one you’d like to try. Share a countdown app and start (or end) conversations with the number of days until your trip. Set aside some dates for planning chats. There are sooo many ways brighten the months ahead with this kind of effort. We do it for every trip and find it adds far more enjoyment to our vacation that we could possibly cram into the actual days!