We used the fittings from D&G - they are just straight transitions with single barbs on each end and a central ring/stop for the tubing between the sections. They were $0.30; tees were $0.50. Since the transition is 2x the cost of the tap, this is why getting 3/16 taps will make this approach a lot cheaper! That being said, I wouldn't try to cheap out on a fitting elsewhere unless I knew it was the exact same source - these went on easily and held really well and as so many of these threads have shown, the results have been quite variable as people figure out which parts work. I was just a really happy customer with what they supplied.
We didn't try this anywhere with lower slope - but I will certainly give it a shot next year if we setup anything with at least 10 feet drop after last tap.
I think the ultimate combo would be to combine one of these diaphragm pump setups with 3/16" tubing and enough control logic that could use the pump to get the flow started and then switch itself out of the way once the sap is running and vacuum is achieved - which would also greatly extend the operating or battery life in a remote application.
My understanding is that the only real shortcoming with 3/16" vs. traditional vacuum is that you don't get vacuum until the sap starts filling the line under gravity flow. The 3/16 would also have an advantage of a lot less volume to pump out to get the vacuum drawn initially to start things up.
At that point, it's not clear to me what else you leave on the table as compared to a high end vacuum system - but I have never worked with maple vacuum systems, so maybe I'm missing something.