Several people have tried sand filters on fish tanks. Typically the sand gets mucked up too quickly and becomes more work than its worth to keep up. I've not personally tried it. I'd speculate the filter balls you posted would be better than sand, but that's nothing but speculation.
Throttling a magnetic drive pump is common and harmless. Throttling a straight drive pump stresses the pump, generates extra heat, stresses the plumbing and results in early failure.
I like trying out of the box ideas. If you do it please report back and let us know how it's working. And if it doesn't work out, you now have a filter for a back yard pond. One more reason for a back yard pond.