I marked my Mac as stolen would it still go on if it’s factory reset?

The Factory Reset will not remove Find My if they do not know your Apple ID and Password. The Mac is locked to you and they will not be able to use it. That does not affect your ability to use Find My.

Your Mac is safe. The thief can't get into the Mac or reinstall macOS because of activation lock. He or she need your Apple ID and password to proceed.

Assuming you set an Activation Lock on your Mac, which you should have done, it cannot be factory reset without your Apple ID and password.

Activation Lock for Mac - Apple Support