I see your issue, its the choice of phone actually. Also if a lot of people don't want to hear that, but if you plan to actually use your phone there are not that many android devices to actually choose from.

I have a G3 currently and it also sucks, before that i always had Nexus phones and with them everything worked perfectly without any issues all the time. Non locked, easy to root, many tutorials and many helping tools.

You need to make sure the manifacturer has a open bootloader. I think only Sony does this most of the time, LG promised to but failed it with the G3, not sure about the G4.

No idea about the warranty tho, in my country i can do whatever i want (terms of software) with the hardware i bought, they cant take my warranty for software changes. But again nexus does not care for example.

Also don't use any crazy roms. Use common ones like Cyanogen or AOKP, no forks from one man teams or shit like that. AOKP is more stable and smooth than Vanilla Android in my experience.