I also think something along these lines is the correct answer. It can be hard to pin down an exact metric because once you start optimizing for a metric it tends to not be a good measure of the original thing anymore. But in general I think it comes down to some measure of feature velocity combined with a counter metric on support/maintenance burden.

"Number of PRs merged" seems like "number of lines of code" wearing a trenchcoat, and I thought we all agreed back in the 90s that number of lines of code was a terrible measure of software productivity...