Why can't you add this to the construct panel?
Because your workaround might be worse. If it is taking several seconds to load the app before Construct is ready to display anything at all, and you disable the splash screen, then the user will open the app and see a blank screen with nothing happening for several seconds. They might think the app is broken, close it, and then uninstall it. The splash screen is vital feedback to say "this app is open and working, wait a moment for it to load".
So long as the loader style is not "none", Construct should be able to hide the splash screen very quickly anyway. All it has to do is load the core HTML and JavaScript file and then it will hide the splash screen and switch to its own loader. I have no idea how that could take more than 1-2 seconds, even on a weak device. So part of the problem here is it's not clear what's actually going on that causes such a long delay on the splash screen.
If you just hack around stuff without dealing with the root cause problem, it often makes things worse, or allows the situation to deteriorate further until it can no longer be worked around, and then you're really stuck. The problem appears to be at least in part that Google are incorrectly measuring splash screen time as the app not responding. This ought to be reported to Google as a bug that they need to fix. The more people who report it the more likely they are to fix it. If they fix it then the situation will be improved and you won't need to do hacks to work around it that might cause users to think your app is not working and uninstall it.
FWIW enabling AutoHideSplashScreen just means "hide the splash screen on a timer". The time delay is SplashScreenDelay, which defaults to 3 seconds on iOS, or when onPageFinished is triggered on Android, according to the documentation. If Construct finishes loading and hides the splash screen before this time, it will have no effect. If you configure this and it hides the splash screen before Construct finishes loading, the user sees the splash screen, then it disappears and they're looking at a blank screen doing nothing again; once again they may think your app is broken, close it and uninstall it.
So I guess you can do that if you want but it doesn't seem like it actually makes the situation better for actual users. Basically you're showing the user a non-responding blank screen to try to work around a measurement bug that affects numbers shown on your dashboard that customers never see. Are you sure you want to do that?