Hydrated lime is only moderately soluble in water.

You may have to get a concrete mixing tub (cheap at big box hardware store) and physically agitate the material with a paint mixer attachment on your heaviest drill. Then suck it into your tote with your 2" pump.

For a bigger job, a better option might be to rent a concrete tub mixer and use that. I can rent those pretty cheaply, and they sure are handy for concrete work. (I don't know how well it would mix your lime, but most have some blades in the tub, so it should provide good mechanical agitation of a heavy material.)

Another thing that might help is to use the hottest water you can access. The solubility of your lime is about 4x higher in boiling water compared to room temperature water. Maybe crank your water heater to the highest setting and use that? That might be worth the effort.

Finally, how finely ground is your hydrated lime? Grinding it considerably finer will greatly increase the surface area and substantially increase the solubility. If you are going to have to do it every year, then it might be worth the investment in some grinding equipment?

Good luck on your pond treatment.