I bought an unknown brand on amazon for 250 maple syrup dollars and I'm pretty sure everything it says on the box is true. It's 1080p native and 450 lumens. I use it to watch movies outside. I have to wait until sunset but otherwise I'm really happy with the picture quality. This is the one I got. I think if you got anything less it would probably be okay for your needs, but you might regret not getting something better the first time.
Genuine question: Why do we want gapless folding phones? If something gets in between the screens (like sand or something) and then you put your phone in a pocket isn't that going scratch a lot easier? I'm hoping to get a folding phone for my next device so I'm curious.