It's crazy to me that you think perceived exertion is the best solution to the running scenario, given what a radical departure it is from your position on cycling.

I bet you can imagine that it's very difficult for triathletes (or anyone really) to believe that cycling and running are so fundamentally different that a tool that can accurately and predictably measure power* is vital in cycling, but worthless in running. The parallels between the two seem way too clear for this to be true.

* assuming this Stryd or something else like it could achieve this