We love Universal, and hope to get back soon. Last trip there was 2018. We can't quite swing the cost of both universal and Disney tickets in the same year, so do them separately. We often do a year of Disney and get APs, and then take off a year, During that off Disney year, we try to do Universal if possible. We always stay on site, hubby prefers Royal Pacific, and I kinda like Hard Rock better. The express pass is a must for our trips. Arrival night is often spent at a nearby hotel, there are several across the street that work well, or near the convention center. We check into the Universal hotel as early as possible, often in the 7 AM hour, and can get express pass starting that morning, and do 2-3 nights.
As far as going if the kids are too short.....I'd wait, unless you know you can go back soon, or if you just want to check it out first. Dr Seuss land is fun, the splash areas are fun (not sure if they're open) , etc. but the 2 main HP rides have that height requirement. But if you know they'll be ok w missing out on them, there is lots to do in Diagon Alley/Hogsmeade. Plus the train, and Hippogriff. The interactive wands look SO much fun (we haven't done it yet, but hope to,) and they can take a bit of time. I dont' know where rider swap is at Gringotts, but if there's any chance they can at least go through the bank area, that would be cool. I think you can go through some of FJ before you have to branch off for rider swap. Not sure if you can see the inside of the castle..... But missing the 2 main rides might be a bummer. we enjoyed having 3-4 days at Universal. We didn't feel rushed, and we LOVE multiple rides. Common for us to do MIB like 5 times in a row, etc. Also gives us a chance to work in ice cream from Florian's, meals on City Walk, or at a HP restaurant and just not stressing over time. and enjoy the pool. Pools at RP and HR were fun.
Universal rides are much more intense than Disney. Darker, and much faster. We do love them. I'm personally very, very glad they did Harry Potter instead of Disney, and we love Disney. I also wish they'd done more with Marvel, and not lost it to Disney. SpiderMan is still one of my favorites at IoA. Needs refreshing, but it's fun.
Good luck with the planning. and congrats on the DVC purchase. we bought in 2011, and have loved it. One more trip this year at thanksgiving to finish our 'year,' and APs, we just came back from a BCV stay, and will be at AKL-Kidani, and BWV in a few weeks. BUT also hoping to get back to Universal sooner than later, maybe late 2021....