One thing to note, if you cancel your Fi service, Google does not promise you that your old Google Voice number will be available for you to go back to. (Even if that is the number that you ported from Google Voice to Project Fi.)

It sounds like you're only considering Project Fi for brief trips abroad. I'd say there's not much benefit to that over simply using Google Voice when you're abroad. If you're mostly in areas where you have wifi, Google Voice has you covered for making and receiving calls and text messages anyway. The big advantage of Project Fi is that you do not need another carrier's service (Verizon or some local carrier where you're traveling) to "support" your Google Voice reception when you're out of wifi. In money terms you'll have to look at balancing the cost of Project service with the cost of Verizon's international roaming or a local carrier's SIM service.