With Re-Shade (at least with the CRT-Guest-Advanced port), you have to tell the shader what is the native resolution of the game so that it will know how many scanlines to draw.
I have never used the Megatron Re-Shade port though.
In some emulators and Core Options there’s an option to downscale the output to native resolution after upscaling.
What you’re trying to do isn’t a standard feature of CRT Shaders or even CRTs though. The only reason scanline gaps are so obvious on 240p content is because the CRT is actually skipping every other line when drawing the frame.
For high res content, no lines are skipped therefore the gaps between the scanlines are a lot smaller.
Normally, in RetroArch, a shader would workout and display the correct number of scanlines based on the resolution output by the core.
What you would like to do is really an enhancement.
Firstly PSP didn’t use scanlines natively at all, unless you were fortunate to use the video output cable on a CRT TV.
It might be a good idea to try out one of the Handheld Border shaders for that system.
Also, the answer to your final question depends on which Preset Pack you’re referring to.
If you’re using my Mega Bezel Shader Preset Pack, you can use one of the DREZ presets if the Core doesn’t support downsampling to a low resolution natively.
PSP’s native vertical resolution is 272p so you should get some good looking scanlines. I think there might be a DREZ preset specifically for PSP in the Mega Bezel variations folder. You might want to try that or you can use the one I included for PSX in my CyberLab Mega Bezel Death To Pixels Shader Preset Pack.
You can use my Mega Bezel or CRT-Royale presets in conjunction with RetroArch’s built-in HDR implementation.
If using my Sony Megatron Color Video Monitor presets, you’ll have to prepend some sort of downsampling shader if you wish to do what you’ve described with a core that doesn’t downsample natively.
How do PCSX2 and PPSSPP look when you run them natively or at 2X internal resolution?
Do remember that my presets, especially the Near Field presets already contain a lot of post processing, filtering and smoothing which work best with low res content so if you upscale, you might actually be missing out on stuff like that.
Most people suggest using standalone PCSX2 as opposed to the RetroArch Cores though. I think it’s called LRPS2 now as well.
Also a picture paints a thousand words so do share a photo or a screenshot so that I can see what you’re describing and more easily assist.