Sorry to hear about your 5x. I had two die in the spread of 2 months, my wife’s in February, mine just a few weeks ago. I talked about it a bit in the Project Fi thread.
Short version, my wife’s 5x (roughly 14 month old) was bootlooping. Had no problem at all getting LG to fix it for free, although it took about 7-10 days to get back. She used an old phone with Hangouts on it as her phone during that time, which worked great except when she wasn’t on Wifi.
My 5x (24 months) would not boot at all, and LG would not fix it because they consider it a different issue. I ended up getting in on the Moto X4 deal, grabbing two of them. We’ll keep the 5x around as a backup in case something happens to one of them. They’re ok phones, although as I recently posted they are a bit heavier than the 5x. I also noticed the other day that yellows on my X4 look more like orange, so not sure if I’m going to need to get support involved on that.
Not sure how many lines you have active on your Fi plan, but my understanding from talking to them is that you need to have both phones active on your plan at the same time to be eligible for the $250 credit, so if you only have one line I don’t think you would get it.
If LG is willing to fix the 5x, I would definitely have them do it. If nothing else you’ll have a backup phone if you need it down the line.