That "system data" is a misnomer. Some of it might be data, but most of it is your operating system and lower-level firmware. It's just that LG haven't described it well in their storage menu.
In fact it's not even just the system software: normally they include the entire size of the system storage partitions in that description, including any free space in those partitions. That is however more reasonable than it sounds: without root you cannot access any space in these partitions, so it's actually realistic to count this as "used" space as far as you are concerned.
Because they live in a different partition, system apps (i.e. pre-installed apps) do not use any of "your" space, so uninstalling them would not free space. However, any updates to them, and any data they store, so use your space. So if there are pre-installed apps that you don't use you can disable them and clear their data: I believe that disabling will uninstall any updates to them so you will recover some space that way. The system should stop you disabling any really vital apps(*), but if you want advice on particular apps you can ask here.
(*) Sometimes manufacturers or carriers make unimportant bloatware impossible to disable. Presumably they have been paid money to include it, but I do not regard this as acceptable.