January is a great idea- it's much easier for my kids with SPD to go that time of year (even besides the crowds, the temps make a huge difference). We also use a similar type of ear muff, and as long as the kids could fit in it, a stroller with a shade that can be pulled down to block things out, and got the "Stroller as Wheelchair" tag, which made a massive difference (one of our kids also has hypotonia). We also get
DAS just in case.
For our family, taking a break in the middle of the day (during the brightest sun and busiest crowds, and to help break up early days and late nights) was a game changer. It used to mean having a two hour long sit down meal in a park during the day, but now that we have a dog, it involves going back to Fort Wilderness for a few hours mid-day to take care of him. One of our kids has said this makes him enjoy the parks more because he is able to really decompress and re-group without constant sensory overload - seriously, staying at a Fort Wilderness cabin, with that calm in the midst of all of the chaos, has been a game changer for us.
We also watch the fireworks at magic kingdom from the TTA people mover if possible or from the beach at Fort Wilderness if it's not a MK day. We have used the wheelchair viewing areas at Epcot before, which was fine, but with MK getting away from the crowds and chaos is pretty key (for us anyway). Our kids actually seem to prefer watching the pre-recorded fireworks show on TV even compared to seeing them in person though so YMMV.
Also, and this should really go for everyone, but staying hydrated (with water vs. soda or milk or whatever) makes a big difference - it's really easy to carry around plenty of bottles of water with a stroller; if you are ever without it though WDW sells these great water bottle holster carrier things - I thought they were a waste of money at first but everyone in our family carries them and stays much better hydrated than when we had the water bottles in backpacks.