I played around with upscaling PS1 games in retroarch last night and I was pleasantly surprised. There are some YouTube videos that talk through the process https://m.youtube.com/watch?v=6yDWYeuQ2pI
It can breathe new life into some dated looking games. Highly recommend giving it a try
IMO, that's the wrong direction. The right direction is to apply some good CRT shaders. Those games were designed for CRT and low resolutions and they can never look great when rendered at higher res. Upscaling works much better for PS2 generation and up.
I didn't think to experiment with the shaders much, so I'll give that a shot. I briefly tried the cell shading shader, but that looked way too unnatural for me -- but it is neat.
(As an aside, I have my original PlayStation 1 and I really want to snag a real CRT at some point for some true nostalgia.)
I have two real CRTs (a 4" JVC radio TV and a 27" Sylvania), and while none of the existing shaders perfectly capture it, a guy who calls himself "Retro Crisis" on YouTube and Github has some modified CRT shaders that come really, really close.
My only gripe is that he has different shaders per system, rather than a single "this is your CRT so all games will correctly render through this one" shader.