That’s not how pain works. Pain is a noisy and error-prone signal that gives you a good approximation for “stop doing that.” But the edge of pain sensation doesn’t indicate what is or isn’t a good idea during recovery. It’s not a bad place to start, but everyone has different pain tolerances, some injuries are in nerve-poor areas, and repetitive stress or tendinitis are definitely not going to appreciate getting lit up right away.

Your edit shows that there is no good rule for the threshold where pain would indicate too much load vs. still being in the beneficial range. We don’t even have a good way to assess subjective pain (one’s 3 is another one’s 7, etc.). “Optimal load” is really just a tautology.