How do independent researchers, doing research after hours, in the evening or the weekend, finance this?

This is quite a good thing, as you will no longer have to buy all the research papers to advance your own research.

The only downside is when you will need to publish your paper, in case you can get closer to a university or organisation to help you finance that or choose to publish in another journal.

I don't, I publish directly on Wikiversity. There it's available to read, use and edit by every follow human with an internet connection. Those willing to contribute with feedback can do so through discussion pages.

Most reputable journals will waive the fees in this case, though the easier route if you are in a rich country where this is less likely is to partner with an institution. They get to add to their research output stats and you get your funding, a win win.

Some journals support “green open access”, where you can share your article minus the journal’s formatting on open repositories etc, sometimes some time after publication, which is usually free. I can’t see any mention of this from the ACM though

You don't :( You look for alternatives. You get discriminated based on wealth

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