Most unions function to keep an established order where seniority matters more than capability and performance.
Besides sports unions, actors guilds, and other small examples, unions exist to provide a framework for collectivising unskilled labor into a unit of power, where individually they have none.
Even unions in industries with legitimately undifferentiated labor will have shift differentials built into their contracts.
https://amp.theguardian.com/football/blog/2023/sep/20/inter-...
What you’re doing is trying to craft a special definition of a union that excludes the most analogous unions to the one that software engineers could form and ignores the real examples of solidarity between professional class and other types of workers. See also the solidarity strikes against Tesla in Sweden. I won’t speak to your motivations in attempting to do so, but it doesn’t correspond to how labor organizing is actually practiced.