Opt-in by default.

Not all unions are great, but the median unionized worker earns more than a non-unionized worker in the same field. So the main reason people wouldn't want to join a union is because they've been conditioned to think they're bad, against their own economic interests.

I feel like you're reducing everything to material conditions. Joining a union would (statistically) help me economically but my objections to being in a union again are all moral convictions. Perhaps workers are optimizing for concerns you do not share?

Yes, I am reducing unions to economic concerns, but I should mention that most people's "moral" objection to unions is conditioned by parties whose interest is solely economic. When the Koch foundation buys airtime on news programs (effectively) to spread the idea that unions are evil and striking is immoral, it's not because they actually care about morals, it's because unions shift the economic playing field in a way that they don't like.

Is your moral objection that you're opposed to improved material conditions?

I believe I should follow my word. Striking violates my principles.

Would it be ok if you were upfront about the possibility of striking? That seems like a very achievable adaptation.

I think that would be acceptable to me assuming I had assurances I wouldn't be asked to participate or be represented by union actions that would be violent. No threats, no breaking things, no vandalism, etc.

Strikes are generally non violent, and the majority of times that they become violent, it's instigated by strike breakers hired by the company (including police). There is over a hundred years of history of this - plenty for you to read if you want to learn more.

The same way we avoid forcing people to support companies they hate. People can always quit if they want.

This strikes me like the people who (after a big contentious election AKA every election) tell the people whose candidate lost, "if you don't like it, move to another country"

Except I think switching trades (which you would have to do since the union is all encompassing and forces membership) is a lot more onerous than emigrating, and emigration is no slice of cake.