Production has and would get moved anywhere else.

That seems to simply excuse capital flight. Some machines might get moved, and financial resources redirected but typically for the most part, the most important element of the means of production generally stays in place; the worker and work floor (and biggest pieces of equipment) tend to stay put.

Automation effectively kills unions

Pure technological replacement is a bit of myth, a deliberately exaggerated bugaboo to keep labor anxious and insecure, it's very rare. It's probably more common for individual roles and functions to be gradually be made more efficient than entirely replaced, stuff like a task being made extremely easy by a spreadsheet once it's properly designed. A lot of IT projects are considerable one time capital costs requiring only a little maintenance.

Innovative technological or procedural efficiencies aren't the exclusive property of capitalists. Have you heard of Co-ops? Do you think they could move any tools or machines anywhere else if a union of workers occupied the factory like they've done in France and south America? Of course they would likely get a police swat team or the army in response in those circumstances in the United States, but automation plays no part in that. It's purely incidental and takes a distant second place to culture, politics, unjust laws and their brutal enforcement.

Unions power is proportional to the demand for labor power

Again you seem mistaken or merely short sighted. A unions power is proportional to the imagination, intelligence, dedication, capabilities and tenacity of a group, perhaps in a word, it's solidarity. Of course one will be weak if it can easily be divided and conquered, if members can be tricked into only considering their wants and needs independent or despite their fellows. It a more general principle too, meaningful beyond just workplace negotiations, but more broadly in society generally.

unfortunately [demand for labor] is now pretty much gone.

This seems like a myth! There would be no races to bottoms, if labor were valueless. It's has been and will always be the foundation of every society.

Workers still need to form militant organizations

Well that's escalated unexpectedly in a strange direction. Probably a poor strategy since most governments retain the only legal monopoly on violence in societies. They should defend themselves, but advocating militancy excuses violent counter reactions.

but individual workplace unions can't do the job,

You seem to be advocating something larger, like the IWW or a union on an industry or national scale. That's a very important consideration.

because they don't really need us anymore in any significant numbers.

Really? Amazon is now among the biggest employers. The more things change...

We're an utterly disposable class of people.

This can be construed as a bit ignorant and a defeatist attitude. Your comment in some ways is a prime example of drinking the cool aid, accepting cultural hegemony. Low skill workers are meant to be fungible, like any other commodity, but not exactly generally disposable (maybe individually). In a society that has abandoned all pretense of social safety nets, and embraces the most vicious social darwinism perhaps then those who cannot justify their existence by work, can be considered entirely disposable, however even some of the most brutal capitalist regimes pay a little lip service to those unfortunates incapable of working. It's necessary for them to not let conditions get so harsh that it completely decimates the lower classes, to keep an army of hungry and desperate laborers in reserve always at the ready. Life must be hard and uncomfortable, but not impossible, that's the recipe for violent revolution.