Actually, believe it or not but I implemented most of them (specifically, texture warping, vertex snapping plus 15-bit color with dithering) the other day precisely because I was wondering how challenging it’d be and it’s literally 15 lines of simple HLSL shader code for about half an hour of total work (from googling the necessary info to debugging, and I’m not a very good shader programmer), and those can be easily plugged anywhere. It requires nowhere near the level of knowledge or effort a NTSC shader does, and you get those everywhere. And in fact at least one person has already made a PS1 shader pack for unity.
EDIT: Here’s the short test I captured back then (using a sample room from Omikron and very crappy camera controls. That black bar at one point is when I erroneously grabbed the window’s bottom instead of the camera):
There is one PS1 visual artifact that’s kinda complex and time-consuming to reproduce, it’s its depth sorting because that one has deep ramifications in the whole rendering process and you’ve gotta do pretty much the whole geometry transform phase manually.
Sonic Mania’s special stages are a different thing, mode 7-style graphics the mega CD was capable of, which behave differently from PS1 3d (they’re perspective-correct). They’re raster-based rather than polygon-based so depending on how accurate you want to get you’ve gotta think seriously about how to approach it.