>I want to be able to promote quickly people who succeed and show an aptitude, and maturity. I want to encourage and incentivize those people to keep reaching for the stars.

Ehhh, you can have this and have a union at the same time. And unproductive slacker orgs without unions definitely exist.

Pay/promo by seniority isn't some innate property of unions, it's a response to a lack of pay increases by management. If management keeps union members happy with their pay, the members have no reason to vote in leadership that asks for pay by seniority and have a lot of reasons to avoid it.