Should I stabilize my 4K footage for use in a 1080p timeline, Before or After editing the footage

I put 4K files in 1080 projects daily, and never do anything to them unless it is necessary AFTER I'm finished all my editing. Leave it to last, and only if you need it. Stabilization analysis files in my experience are not very big at all, negligible. It's the AI and ML stuff that's gonna eat up drive space.

Analysis files can be very large. Why expend more space and time than you need to. Edit first.

If you have that much footage, set Import settings to Leave files in place. Also disable Background Rendering in Settings. If your final output is 1080, then create your project as a 1920x1080 HD at a frame rate that matches most of your footage, unless you have a specific output frame rate you need. The 1920x1080 project will "downscale" your footage by resizing it either to Fit or Fill, depending on what you set for Spatial Conform for the footage. As Tom says, once you've got your edit cut, then go back and stabilize the shots that need it.

If you're new to FCP, here are some free tutorials that are recommended around these parts:

Steve at Ripple Training’s 60 minute FCP tutorial overview https://www.youtube.com/watch?v=HqUP7Zgeuck

Final Cut Bro's getting started  https://youtu.be/L3bZ-ETKZ8U

Izzy Video tutorials. Older so the UI looks different but the concepts are still correct. https://youtu.be/_jvyrnQpR1E?list=PLp5SAgbM8S87n1qvv34-PAU2zMHLlXyfb