Customized browser start pages?
I use Booky for bookmarks so my startpage is booky.io, but I have a userscript to remove a bunch of unnecessary padding and hide the header and footer. They're toggleable by clicking on the collection name.

Also, writing userscripts for React apps is a pain in the ass. Currently I'm listening to click events on the body and reference-comparing the target to the result of a query selector. This is about the best technique I could think of that is easy to use, works every time and doesn't require me building a debounced mutation observer in a userscript.