Not just reduce the monitor brightness, but also use "dark mode" whenever possible. I read a lot on the web, and found the darkreader extension incredibly useful for my eyes strain.
Dark mode is effectively the same as lowering brightness. If you can lower the brightness to match dark mode's light intensity, it should feel about the same.
There are some sources suggesting that all other things equal, black text on white is more legible too.
There are some sources suggesting that all other things equal, black text on white is more legible too.