And that's exactly why I will never join a union. As soon as that happens at any job I have, I will hand in my resignation the next day.

I can negotiate very well on my own, as opposed to being stuck into seniority "levels", thank you very much.

Plenty of unions and associations don't do this. A reasonable method for this would be for a union to negotiate per level salary bands, and yearly merit increases.

Within the band the company pays whatever they think you are worth.