As you can move it around on level ground, then the dinghy and trailer is not too physically demanding for you.
Pulling up a slope is hard work and is really a two person operation.
If that is not possible, then you need to use your towing vehicle or utilise a pulley system. A jockey wheel will help in either scenario.
Pulling up a slope is best done in one operation to keep the dinghy and trailer moving. Once you stop, you will require a lot of physical effort to overcome the inertia/gravity to get moving again. If you need to stop, then rigging some form of automatic wedge behind the wheels, to stop the trailer rolling backwards may help.
If the beach area is shingle or sand, then wider/bigger rigid trolley wheels may help, rather than the standard pneumatic type.
Just some ideas for you to ponder.