In a monorepo, path filters alone are not enough. You want an affected graph (packages and tests), fail-closed when the graph is unsure, and a canary on main that still runs the full suite. Unsure means run it. Unsure means skip it is how you ship a lie.
I would not start with a new product for this. Instrument where minutes go (checkout, setup, build, test), tighten the trigger graph, then decide if anything is still left.