Man, that is really going to annoy the wildlife…
Just to backtrack here, isn’t the main thing to get rid of the trash? I’d start by raking out all that’s easy to lift (maybe start with a pitchfork) and letting it settle, maybe two or three times. Then use the pump just to vacuum up the last of the litter. That would save emptying and refilling several hundred gallons. (BTW, is this a concrete tank or a real pond with a mud bottom and all? If it’s a real pond without a waterproof liner, you probably can’t empty it.) If your wife wants it scrubbed, that’s a different matter.
It’s man-made and vinyl lined. I have been hesitant to use a metal rake or pitchfork because I am afraid of puncturing the lining. Just using the skimming net is approximately worthless and it just churns things up alot without collecting much muck. Perhaps gently using a metal rake will be worthwhile however.
If you have some money to spend, there are pool filters available for water gardens. I had a small one that filled up vey quickly and gave up on mechanical filtering.