If you're willing to splurge for a deluxe hotel with EP I would go for it and go during March. August is brutal. The weather gets more brutal each year. March should be fine, but moving even to just April, or May, can get some heat.
For Florida, Winter only lasts a couple of months and "spring" weather might come every other weekend, or a few weekdays, or so, if you're lucky during Spring.
While all the perks on staying on-site are amazing, even if you didn't and stay off property to rent a home and you do not mind waiting as you previously stated; the good thing about Universal is that almost all their queues tend to make plenty of room for you to still be indoors. Disney is fine in that department, however Universal I feel is far better in designing and ensuring their queues are indoors and lengthy. The few that are outside queues are water based rides, or queues in which you'll quickly gain access inside.