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Try option-click on the green icon in the app window. It’s well hidden and unintuitive, but I think it does what you want.

(bonus: this is not exclusively a Safari feature, this is a macOS feature)

option click does not fully maximise!

I just use Rectangle. It's got all the core functionality. https://rectangleapp.com/

Fn-Control-F or hover over the green dot and pick the maximize option

omg it works.. why is this so hard?

I hate this default behavior on the green button. I can count the number of times I've intentionally wanted a full-screen app on one hand.