Use the Trailer and the Launches. In the end it will be quicker, cheaper, and fewer problems. Building a 'Dolly' is really building another Trailer. But if you insist on trying to shortcut it, why not use your existing Trailer, with the 4-wheeler?
The launch on the upper right lake is useable, but because of the recent flooding, the trailer parking area isn’t, and won’t be repaired for 4-6 months. So it’s ~half-mile round trip hike to the truck from the launch.
My MO has been to dock at the launch, hike to the truck, drive to the launch, trailer it up, pull to lake #2, launch, dock, drive to the parking area, hike back, go fishing (Bowfishing lately.)
The pontoon is an ultralightweight, and I can easily beach and drag it completely out of the water by hand.
The soft beach (sandy soil) doesn’t lend itself well for backing the trailer down it without tearing it up. Also, pontoons sit very high on trailers, (no keel), there’s no way to trailer it up when out of the water.
I think I’ll build a simple wood t-bar dolly with wide, short-height tires I have from my boys kart racing days. Adding a electric motor from an old trolling motor, chain and sprockets is a thought too.