How can I never see scroll bars? I can change the scroll bar behaviour to only show when scrolling:

 > System Preferences... > General > Show scroll bars



But I never want to see scroll bars.

How can I completely disable them? I just want to scroll with keyboard/trackpad, the scroll bars are ugly and get in the way too often, and I never have a need for them.

I want to turn off scroll bars 100%.
You get the three options shown. That's it. macOS makes no provision for your particular desire in its design.

You should make an enhancement request at Apple's Product Feedback site.