Q: How do I change Preview default settings for scanning?

There doesn't appear to be a way. If you monitor the "More Like This" box (top right), other threads appear. Opening them usually displays other threads. The question has been asked a lot with no solutions.

Send Apple feedback. They won't answer, but at least will know there is a problem. If enough people send feedback, it may get the problem solved sooner.

Feedback

If you use Image Capture instead of Preview, which uses a very similar interface, then the settings you make in Show Details pane of Image Capture will be the deafult settings used the next time you use the application.

Thanks PAHU! Worked for me!

Brilliant Pahu ta mate. Can't believe I have been using Preview so long & changing the settings manually each time.

For some reason this didn't work for me.  Elegant idea. 

Closed Preview.  Opened Image Capture, set new settings.  Closed Image Capture, and re-opened.  Settings stuck.  Good. 

Opened Preview, old settings persist. 

You might want to consider starting a new discussion. Since this one is marked solved, less people are likely to look at it. You can link to this one.