We used to divert ours with a stone placed in the flow. That allowed the flow to cascade down the rocks instead of shooting into the pond.

If you install a ball valve after the pump you can adjust the flow to your taste and always have the option to change the flow at any time. A little work but easier than always fiddling with rocks, etc.

Ball valve - hmmm that would help in other ways too (ie would be good to slow it slightly through the filter set up) Does it harm the pump motor in any way to slow it - would not be a drastic change.

Never restrict the flow into the pump, but regulating the flow after the pump is fine.

If you can afford it, buy the best ball valve that you can find. Cheap ones turn balky trying to turn the handle. I found expensive ones always work smoothly and you can adjust the tension.

Could you modify what the stream of water hits when it leaves the pipe? Like a stack of rocks, or a series of levels that allow the water to trickle down several paths before it gets to water level?

That's the design or creative side of things. Two things come to mind based on some problems I had. First, every time you increase or decrease the flow with the valve, you will change the look and sound of the stream of water, so finding a balance between flow and landscape will take experimentation. Second, think about adding a fitting of some sort at the end of your pipe to help create what it is that you are looking for. Add something to flatten the water flow out of the pipe. Or something perforated to create little jets of water.