There is microg which can replace GMS. But we really need an alternative, not based on some company's shit.

But it's based on a browser that's not made to be secure, but instead to have the most features and comply to all these standards. So removing them will make it a bit more secure, but it will never be good. The best browsers are the ones that aren't made to support javascript and all these other standards. A private browser would be something like w3m or links. Ideally, it wouldn't be HTML but gemini's gemtext or just markdown.