A little update.
I tried running both pumps letting the canister feed the new pump.
It work pretty well, but after further research I found out what a disaster it would be if one of the pumps would fail.
Since the problem is purely after the canister filter I got the idea to use a finer prefilter. Had this running all night and I could barely see any particles in the filterfloss in the first reactor.
So I removed the reactor which improved flow a bit.

I spend most of today, rearranging stuff and added my 350 gl/ hr eheim canister which has heater in it.
Some piping, hoses and a lot of filterfloss, I had it running.
With heater in the canister I can swap the other inline heater with my UV light!

Both the fish and myself, need to get used to the additional piping in the tank, but I much happier with this simpler solution.
And running two filters also adds a safety net.