3/4" for the discharge... I suspect that you're right, I don't think I can move 1l of water through a 3/4" line in 2 seconds. Definitely going to larger pipe for the outputs if I need to.
The air pump idea reminds me of something called a Reverse CSD, which uses air pressure to push water out of a chamber, when a certain pipe gets exposed by the falling water level the air pressure equalizes and water rushes back into the chamber, producing the surge. I can't find the page where this device was discussed though, so I don't know how to build one
I wish I had bookmarked that page.