I'm not trying to be mean here, but if I'm reading the meaning of this post correctly, it feels like you really haven't dived that far into open source. There are thousands of FOSS projects that do exactly as you say, and yes, some get branded and bloated.
But like... that doesn't mean that what is out there needs to strip away anything. It just means that you have to keep looking and possibly contributing even if its just reporting bugs.
For example, Firefox. Have you even checked around? Falkon, Qutebrowser, Ladybird (still in alpha), Nyxt; there's a handful of QTWebEngine browsers already doing just fine. Not to mention the plethora of stripped down Firefox forks for both desktop and Android like Fennec, Ironfox, Floorp, Firedragon, and Zen. There's also a stripped down base Chromium browser, which I believe is de-Googled.
I'm just not quite sure what you want to achieve here.