Park Hopper is pretty important, as sometimes, switching parks partway through is more enjoyable than just one park/day, especially with the weather.
We went twice but it's been several years. First time we stayed off-site and the second time, at a Disney Hotel. Even though they are pricey, we enjoyed the on-site trip more. We went in December though, on the second trip, so the hotels and parks were decked out.
I don't think that for 8 days you need a "day-off", unless you really want to enjoy a water park. Not sure if you can fully experience Disney and Universal in 8 days, so I'd just pick Disney this time. It depends on what you want in it all.
They used to have a Pirate Adventure that costs extra and we felt DGD was too young (she did other extras), but it looked like SO much fun. So, check into the "extra experiences" if you think your DC would enjoy doing one.
Disney can be wonderful or horrible. I think planning, yet being flexible, helps as you can see what all your kids might want to do. Character breakfasts, the dinner show, etc. I loved both trips, even though I am jinxed. First time we went, I ended up quite ill with a kidney infection (they wanted to hospitalize me) and the last time, severe strep throat (first strep infection since 1976). My DGD is too old to go now, but would love to try one more time (as long as I can skip the "It's a Small World Ride"--😜)