I think you are thinking on the right lines. However, as someone who managed tech teams working on products I can tell you it is extremely hard to get these right. You would want to come up with something that you are going to stick to no matter deadlines, sales and marketing pressures, special requests from customers.

Do not get confused between Agile/Scrum best practices with managing people. I have seen teams struggle with estimation and that really leaves the appraiser in a state of confusion on how to judge performance.

Sit down with you team, state your goals - improved customer happiness, reliability and innovation are good enough - and come up with a few metrics, put them in the system for a month or so, review and improve. Do not write this in stone, because each team is different and so is each company.

All the best for a successful product backed by a happy team!