> What are my options? Open the developer tools and tweak the CSS every time? Try to find a good browser extension that solves the problem? Write my own? And some of these solutions are inconvenient or impossible on mobile.

I wrote a bookmarklet a few years ago that doesn't always work, but does work most of the time.

  javascript:(function() {
      document.querySelectorAll('p, li, div').forEach(function(n) {
          n.style.color = '#000';
          n.style.font = '500 16px/1.7em sans-serif';
      });
  })();

I find this a good compromise between "the original design" and "reader mode", which I find often changing too much.

Biggest downside is that it doesn't work well for "dark mode" websites, where low-contrast seems the most prevalent, but I find "dark mode" very hard to read even with good contrast so I typically tend to just not read those sites unless I really need to.