For whatever it is worth, I have two separate and independent bilge pump systems. One with the largest Rule pump available ($99.00 in the late 90s.) Initially I installed a check valve, but subsequently removed it, and simply put a hole in the top of the tubing just under the lazarette cover at the top of the loop.

Then separately I install the Whale with the outlet in the lazarette, well above water level. The output goes into the well and drains from the well - no check valve, and no vent - although there is a loop.

I haven't come across a better system - and it is not because the system was the result of my conclusions. On the other hand, although I do still take on water, I have never had a near occasion to visit my Maker since these were installed.

You mentioned space. There is plenty of space. The 1 1/2" hoses pass in the bilge under the floor under the cockpit and up opening adjacent to the rudder post, then up into the bench lazarettes, one on either side. The starboard one for the Rule pump goes through the bulkhead at the top just under the deck. The Whaler one goes to the Whaler mounted at on the side of the bench lazarette at the aft end of the cockput, as high up as I could get it. That output goes through the bulkhead and the output is pointed down. but yes, it does not go through the bottom but just into the well - no siphoning is possible *-unless the boat is in really serious shape with the decks awash. Then I can rely on the check valve in the Whale.