For individual websites on iOS, click the "aA" icon:

  Website Settings > Show Reader Automatically

For every website on iOS:

  Settings > Safari > Settings for Websites > Reader > Other Websites

Firefox reading mode? It won't work with all sites, though. A lot of large sites are insanely broken, so Firefox doesn't propose reading mode at all, or nothing loads.

Honestly, I have only one way with dealing with those: not using those sites.

I can make this in couple of weeks, but I need an incentive other than just having fun