We use a City Mini GT double. Love it. Super easy to fold up. Just grab both hands (one in each seat) and lift up. It’s really not terrible on the bus on the way there. On the way back, I would try to pack up your stuff while you wait so that you’re not trying to fold up a stroller with all your junk spread out in it. I’m a big fan of 2 gallon ziplock bags for our stuff, carried in a backpack. Makes security super easy & easy to find what you need.
Have you been to WDW with kids before? I would try to build in some pool & resort time if you can. Even if they don’t nap, it helps them wind down I think.
Re: timing of Minnie vans- it depends on how close one is. There is more demand than supply IMO, especially at peak times. By the time you get a Minnie van, it shows up, the driver installs both car seats, you get everyone loaded, etc, it won’t be a speedy process. I think Minnie Vans are great is certain circumstances (resort to resort, early AM travel, late PM travel, etc), but the busses really are not bad. Promise.
If you do use a Minnie Van, please check that the seat is installed properly. When they rolled out this service, one of the big plusses they advertised was that CMs were specifically trained in car seat safety. Unfortunately my one experience with this was that the seats were 100% not installed properly.