I have limited concrete work experience, but I have observed some terribly weak and flaky concrete where guys tried the "dry bag" wall and in other circumstances.
I have done dry Quikcrete mix for 4x4 fenceposts and had zero problems. However, I expect the cured concrete still had cracks in it deep underground. In that application, the cracks are no harm, because you still couldn't pull the fencepost free (unless the cracks created a perfect upwards bevel).
I have mixed small batches of concrete in a shallow mixing tub with a concrete hoe. (The tubs are cheap at your big box store.) I have also rented the rotary mixers for slightly larger jobs. ($50 for the weekend?) You either need 120VAC at the site, or your own generator.
As to monitoring the water level, I couldn't remember if you lived at your pond or not.
Do you have a game camera with a time lapse feature? If so, set it up to take photos every 8 hours from a location where you can read the water staff in your pond. That should get you good data over a month where hopefully, your pond fills and then goes down a few times.
