If you want to make deep and lasting changes to your device, you need to unlock the bootloader. Not all phones can be unlocked, but you can add the LG G5 to the list of those that can. Well, the unlocked international G5 (H850). It's now supported by LG's official unlock tool.
Unlocking the bootloader on your G5 will let you flash anything to the device, like a custom recovery for installing ROMs. However, you will void the warranty doing so. If you've got a US carrier device, you can't use LG's unlock tool. You need the H850 device model (the unlocked H815 G4 was the only device previously supported).
Unlocking the G5 (or G4) is similar to Motorola's system. You need ADB and Fastboot working on your computer. Check the box for OEM unlock in the dev options, enable debug mode, and you're ready to start. LG's online interface will have you input your IMEI and device ID, which you pull over ADB. The site will generate the key needed to unlock the bootloader. The entire process should only take a few minutes, but there is no way to re-lock the device.