Things I'd consider in addition to the two excellent items you mentioned:
3) Negotiating better IP control. Side projects, open source, etc. are locked down at various companies.
4) Better treatment for oncall. I know folks who have had to work for 17 consecutive hours for an incident and got nothing.
5) Better treatment for crunch -- tech workers in games have this particularly bad, but it's not limited to just that industry.
6) Salary minimums
7) More control over how the company outsources/contracts with workers
8) All sorts of other side benefits that could be collectively funded -- union libraries, training, networks, publications, resources for new professionals, discounts on union developed software, etc.
9) Parental leave, sick time, and other basic benefits
Edit:
10) Ensure and oversee processes to protect people who end up on the public stage. If you believe cancel culture is a thing, maybe having a union rep on hand and an established process for managing firings would be in your best interest.
11) Forced arbitration is another thing I'd consider talking with a union about.
So many people have this notion of a labor union built on what it means in an industrial context. But there's plenty of other models that could be applied where you don't have to, for example, promote people based on seniority. I wish it was easier to talk about this stuff without people hand waving away the idea immediately.