This might sound odd, but what happens when you suck (literally, with your mouth, suck the end of the hose) the water out of the return-hose so that there is no longer any air in the hose? Like siphoning something. If you can get a proper siphon going, it would work such that the water coming out of the hose will have a pull effect on the water going up the incline to return to the tub. If you can just get it started, it might continue properly.
Aww man I don't want mold / contaminated water going down my throat...D: But I did try with a test pump by plugging to it and suck... both pumps running; one pushing in water, one sucking out water at the return-hose. The water was flowing better but not too much of improvement. :\
So I tried this. I took the test pump and connected directly behind a new pump so it runs in serial =
Double Pushing Powa .. The result is that water flow did increase, but still not as good as what it used to be the first time bathtub cooling was setup. :\
So I said hmmmmm..... let's try something! I unplugged the new pump from the hose that's going out, and plugged to the return-hose to push water instead...
HOLLY CRAP it pushed water so hard it was farting (trapped air) like hell at the other end of the hose. The water was flowing at full blast, much better than when it was setup the first time in tub. While it was farting a few dead bugs and other debris came out of the hose. It's like you take a dump after you ate wrong food. I went to check my PC CPU temp and it dropped down to 26C in just seconds.

That's wrong direction of flow though. According to my blocks it's supposed to flow the other way around.
So I let it flush for a few minutes. Water flow was solid, I didn't see any more air coming out of the hose.
I then pulled out the pump and put it back in the other hose like the way it was set before. What do you know. It lost water flow right away. It's a little bit better than before, but still the water flow is too low.

So I plugged the pump on the return-hose again, and I was getting high flow rate. Repeat the cycle with same result. What da hell is going on here....
I will take video of this today and will try to post it tonight, or tomorrow morning at the latest.
Could be the physics of it then. To pump the water out of the hose, it would have to push the water up an incline, at the end of your 80 ft run of hose & around several tight corners, including inside the water blocks.
That's what I thought too, but when reversing the direction of the loop the water was flowing at very high rate! So apparently there isn't much restriction in the loop.
cheez