Maybe try planning an ideal day at a smaller zoo/one you know well. I have a lot of your same issues (or all, maybe?) and I've found smaller places are good for experimenting with different clothing items to keep me cooler, what to pack and how to carry it all, how to pace myself and when to take breaks, etc. Like if a certain type of water bottle is easier for me or too heavy, if I want it in my bag or in a separate holder or clipped to my bag, those sorts of things. It can also be better for testing out if things like tablets for IBS are good to take (some will dehydrate you, which isn't good for being out all day) before you go, take during your visit, etc. Think of what your ideal visit would look like *within your control* and then work on steps you can take to make that happen.
Not sure if you do this or not but I obsess over maps beforehand, too. I try to know where all the bathrooms are, where the quiet areas are, if there's multiple first aid type places (bigger zoos sometimes have a secondary one). A lot of places are now putting an accessibility section on their website which can give some helpful info! It varies a lot but sometimes there's maps that show what the noisiest and quietest areas are, if there's a lot of elevation changes, where benches might be.