Could you try editing what you want to on your phone? For example using a file explorer, navigating to the build.prop allowing root access and open it in some form of note editor, usually included in a file explorer app and then try it. You probably get a bootloop because the build.prop you put in has incorrect permissions. I have made many changes to mine through this method and haven't (yet) had any issues. Though I do understand if you are making many modifications it is hard on your phone so you could try editing on your computer, pushing to SD card, moving to system and make sure the one you replace with has the same permissions.
Sent from my ST18i