Possibly not toO late then, but more of a rush & expense as I’d need to get the appropriate lengths and elbows etc. I think the 19mm irrigation tubing will be sufficient as long as it’s suitable.
By the time you buy the extra fittings needed at each path, eg irrigation joiners, clips etc, the waste pipe is probably the same cost. But when you use the waste pipe, you don't need the extra fittings that will just reduce flow, the more you add. With the waste pipe you can run a single length of pipe with no joiners, at a later date. You don't need elbows for the waste pipe. Buy what you like though, just giving advice based on what I did. Made for running the irrigation later nice and easy.
Literally just run a straight lenght of pipe under the paths so that it comes out 100mm either site. No extra fittings needed. Then when you landscape, pass the irrigation pipe through the duct and you're sorted.