Install Mac OS on old Mac Mini not working from USB nor from internet
First step is to erase the non volatile ram (used to be called PRAM) by holding Command-Option-P-R and listen for two boot chimes in a row then release the keys and try the install.

For an erase install, you need to boot to the installer and then choose Disk Utility from the Utilities menu and literally erase the drive. I would physically disconnect my backup if any before doing the erase. At that point, NVRAM or not - the system can’t boot from the erased drive and will make your life easier to select the correct installer and get a clean new OS.