I don't have any experience with these types of pumps and filters but many of us use one pump and combine filters on the
return back to the pond. It's hard to see but is the box on the far end the waterfall return? Could you possibly pump the flow
to the Black Box filter and have the output flow through cascade/spillway and back to the pond instead of feeding it with another pump?
I don't know how much head is created with matting or whether that could be removed since the Black Box filter is before it.
The skimmer looks pretty small for that much pond surface area but hopefully it'll help.
Since funds are limited I think this will be a learning experience and when/if the renovation happens to go bigger, you'll have
a good handle on what works and what doesn't.