Yes, that is how you disable SIP. Otherwise, I'm sorry, but you are the one out in the deep end here. You can't just throw out "this doesn't work" and leave it at that. I don't care how big a font size you use.
If you are claiming that disabling SIP is ineffective, then that is something that someone else could test, verify, and maybe provide a workaround. But you will have to provide more specific details and, ideally, an easy-to-run test that reproduces the problem. This is something that should work. If it doesn't, that would be a bug for Apple to fix.
It is possible that just disabling SIP alone will not disable Gatekeeper. But I'm sure you will have to disable SIP to make further changes that will disable Gatekeeper. You will have to scour the internet for such suggestions, because this is not something that people normally do. Don't be upset if they don't work. I just found one suggestion on how to turn off Gatekeeper "defaults write com.apple.LaunchServices LSQuarantine -bool NO" but it was posted in a Sierra forum. It may not work on Mojave or Catalina. You can expect that any software update, no matter how small, will break your system.
Again, I have to question why you are doing this. You said "it is necessary to test the Mac specific dev builds on a MAC that are not notarized". But current Macintosh computers and the macOS operating system requires notarization. Therefore, your statement is invalid because you aren't testing on a "MAC" anymore. You are testing on your home-grown, customized operating system. There are people around who will help you do that and would find such a task interesting. But you probably aren't going to find them in this forum. I think that Apple would agree with me an acknowledge this as a bug, but that doesn't mean they will ever lift a finger to fix it.
At a real, fundamental level - you are on your own here. Rather than exploring the limits of this discussion forum's font sizing options, I suggest you spend the time to learn about how the operating system works at the lowest levels. Maybe buy the series of "MacOS and iOS Internals" books by Amit Singh. Learn how launchd works and try to figure out exactly which tasks are responsible for these security features you want to disable.
Good luck!