There not being a "clear data" option for this app is normal. Storing 6GB of data is not.
I think this thing should be the app that stores SMS and call logs. On my phone, which has an SMS database of 10.5k messages, this takes 12 MB, which is why 6GB raised my eyebrows. The only thing I can think of is whether you have a lot of people sending videos via MMS (video being the most space-hungry medium available). What happens to this 6GB if you use your message app to remove old video messages you no longer care about?
I don't know which "Messages" app you are talking, but if it's the Google app then yeah, it has very limited settings, as Google apps tend to. But given that on my Pixel, where that is the pre-installed system default app, I can disable it or set another app as default, I can only say that if you don't have those options that's definitely a choice that either LG or your service provider made, whichever of the two installed it (since apps that can't be uninstalled are generally installed to system that means a system update added it, which means either the manufacturer or the service provider was responsible). For information the other case where you can't uninstall an app is if it's set as a device administrator, in which case you can unset that, but not allowing other apps to be set as default sounds like something that was done to to the ROM (because you should always have the option to choose a different SMS app, that's a basic feature).
