It looks like they are suctioning the sludge out of the pond by their method.

That type of work on a small pond is MUCH more efficient (read cheaper) with a long-reach excavator. Your pond is so small, a contractor with a medium-sized conventional excavator could clean out your pond in 1/2 a day probably.

He will be dumping piles of sloppy sludge. You need some place he can rotate and dump. You then need to spread and let dry (for a long time). Then you can move that material to gardens, etc.

My (non-expert) advice would be to keep evaluating your pond expansion plans. Once you make a decision, most of the work could probably be done with just an excavator if it is only a small expansion. Clean out your existing pond at the same time.

Glad to hear you have 50 GPM! For your expansion/clean out project, you will need to build a siphon or buy a trash pump with capacity of more than your spring flow rate. All of that type of work is much easier the drier you can make or keep the pond basin.

Have fun!