What can I do to let someone test my iPhone 12 Pro Max without going through my stuff

Solution 1:

They are buying the hardware not the software. They should be willing to accept demonstrations of features like:

1. The camera (video and photos)
2. The microphone and speaker (make a phone call, play a movie)
3. The screen is defect free
4. The buttons and ports function properly

For all of these, you can retain control of the device. There is no reason to expect they would gain any value from having full control of the device.

If they do want to handle the device, you will have to accept risk if you don't wipe it. There's no way to allow them full access to the device without also allowing them full access to the device.

If they are looking to understand that shape or size iPhone, but don't need to interact with it beyond that, you could look up Guided Access. This would allow you to lock it to a single app (like Safari) and let them play with the form factor.