The first few years this is going to be packed if they put popular IPs in the park. After that they can play catch up with some of their other land and have Mears bring them to the park if necessary then down the road build a garage as needed. I think the 5K will not be enough the first few years but in the longer term you are correct that day guests will come from Universal's and partner hotels and will be bussed in.
Any plans for a huge bus drop off yet?
There should be plans for a bus drop off that can hold at least 10-15 slots just for Universal transportation. (5 for the current on-site hotels; Aventura and Saphire share one, 2 for CityWalk-New Park transit, one for Endless, and a few more saved for the upcoming hotels.)