That would leave the job either half-complete, done by an overworked person, or available only to independently wealthy people.

Meh, I don't think so. You probably couldn't pick a random union member but there's gotta be someone smart, driven, and mad enough to do it for free and do a good job at it. Hell, if I had the skills and resources to, I would.

You are advocating for slavery, unpaid work.

If you willfuly agree to unpaid work, you will be such a bad union leader, don't bother.

You missed the entire point.

Doing something that helps you and helps your similarly-situated fellow worker seems like something that would qualify you for a union leadership role.

Further: People do all kinds of unpaid work to make their community or shared experiences better without (most of?) us accusing them of advocating for slavery.

Slavery implies force. What I said explicitly required volunteering.

That's like calling volunteering slavery. Am I a slave for working at my hospital in the summer?

Unpaid work isn't slavery, it's volunteering. It's only slavery if (for example) you're locked up in prison and beaten by a guard if you don't do it.