When I started with these retro handhelds
It never used to matter to me to simulate hardware screen effects. It was just one of thousands of settings (integer scaling? bilinear filtering? wtf?) that I had no interest in changing from whatever system default was provided.
I'd watch retro game corp videos and boggle at the kinds of minutiae that you could geek out on.
What changed?
It's been fairly recent - playing through Link's Awakening on the RG35XX the GBC graphics were just feeling... crude? Maybe one of the walkthoughs I was using had an LCD screen effect on, or something, and I started messing about with the shader settings.
It really improved things! and the effect was comvincing enough that it felt significantly more atmospheric. I don't want to say "nostalgic" because I think about the only game I ever played on the GBX was Tetris, but still, maybe there's a Proustian element at work.
Well, I suppose that broke the seal for me, so when I was messing around with the screen settings on the RGB30 I explored the CRT shaders.
Comparison
I'm not 100% satisfied with the shader settings I have: they feel kind of excessive. I don't want to play the game through something that feels like a fisheye lens, or that's blurry enough to simulate my short-sightedness.
But I landed on zfast_crt_nogeo.glsl from the CRT shader list in retroarch and it's pretty close.
Screengrabs don't get the shader effect, but hopefully this will give an indication:
Well, that just looks awful so you'll have to take my word for it that it's worth it in the flesh 😂
The CRT line effect is a bit much, but I love the rounded corners and vignette dimming along the edges.
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