How to factory reset Mac without reinstalling from the Internet or from USB? (Mac OS Catalina and earlier)

Install macOS to a drive, then use Carbon Copy Cloner (CCC) to clone the entire drive to a read-only .dmg file on an external drive. Then you can put the Mac you want to "reset" into Target Disk Mode and connect it to another host Mac with full admin privileges. Use Disk Utility to erase the whole physical drive of the system you want to "reset". Then use CCC to take the .dmg image you made and transfer it to the Mac that is in Target Disk Mode. There are some limitations to this as you may encounter issues doing this with a T2 Mac as the T2 security chip will most likely retain the authentication information from the old system which may cause weird issues. This procedure will easily work for any 2017 or earlier Mac as long as it does not have a T2 security chip (I don't know if a 2017 MacBook has a T2 chip, but no other 2017 Mac has a T2 chip). AFAIK, the only time the information in the T2 security chip is reset is when you reinstall macOS or when you "Restore" the T2 firmware which then forces an Internet Recovery Mode reinstall of macOS.

As far as I know, you can either get macOS from the internet or an external storage device such as a USB. The latter is probably a better option. See if your workplace is able to make an exception to the USB rule for this situation.