It is possible that this is a problem that will solve itself. I think a lot (most?) mainstream media outlets are hemorrhaging money, and the gravy train can't go on forever. We'll reach some maximum of Terrible Crap, and it will peter out, and then maybe Google can get back to finding honest content and playing merry hell with Internet standards.
The problem as I see it is that the mainstream websites are not good. Search results that gave a broader range of hits than just that sort of thing would be much, much more useful.
If I want, for example, to find what laptops people consider the best, none of those sites help me.
There are sites that do some good gear reviews for relatively specialized equipment--especially not gadget/electronics. But this isn't the 1990s when PC Magazine would have a 600-page issue with a big chunk devoted to the best printers as evaluated by their on-payroll staff.
I'll occasionally put a review of something up on my site but I have neither the money or interest in doing multi-product comparisons. That's pretty much impractical outside of something like Wirecutter (which I generally think does a pretty good job).
But if these are the complaints about Google - "I only see sites that are so good that they constitute the mainstream" - I'm... ok with that? That seems like a good outcome to me. I don't mind using that tool.
From what I understand of your complaint, sometime in the last 20 years, the Google stopped finding outsider art. I would guess that's due to SEO. And with anything that's known to drive revenue to a business, that sort of thing becomes a target. So people target the Google algorithm to place better. I don't know that there's a solution to that. But I don't think it's because of a change inside Google - more like a change in society.