I’m very nervous about erasing my old iPhone 8 without affecting my new iPhone 12 Pro Max
What is the process to erase my old iPhone 8 without affecting all data from my new iPhone 12 promax
iPhone 8 Plus, iOS 14
follow the support article at the top of the page under Related Article. It won't affect your new iPhone.
If you are really worried though, make a backup of your new iPhone first.
How to back up your iPhone, iPad, and iPod touch - Apple Support