[neebz]

In Safari you can set reader mode turned on for specified websites. I have done it for all the sites which I visit frequently. Unfortunately there is no way to to turn it on at global level.

The only minor gripe I have is the slide-above animation when reader mode turns on. It's jarring when you keep getting that on each navigation.

[Jakob]

You can turn it on globally by selecting:

Preferences > Websites > Reader > When visiting other websites: On