Need help with reshade for visual impairments

New user here. I posted on reddit a couple of days ago about making games more accessible for my 5 year old son who is visually impaired and quite a few suggestions were to use reshade.

He is very much in to sonic but it’s way too busy for him to focus on the obstacles. I wanted to see if there was a way to remove backgrounds completely so that I’m left with the character, floor and obstacles. Suggestions were that this is possible with reshade.

Have installed it and gone to settings in game but don’t fully understand what I’m doing.

This is for Sonic Mania by the way.

Which one of the settings would allow me to remove the backgrounds?

Help much appreciated.

The "Chromakey" effect that comes with ReShade allows you to do that. In the ReShade settings, search for "Chromakey" on the home page and enable the checkbox. In the bottom half of the window you can then change how far away the background removal should start at and which color the removed parts of the screen should have.
For this effect to work ReShade needs to grab the game's depth buffer. While this works in the majority of games now, there are still some where this does not work. I don't know how it interacts with Sonic Mania. Seeing that this is a 2D game, the chances of it working is pretty low unfortunately. To make sure the depth buffer is available, enable the "DisplayDepth" effect. The screen should then turn into a black and white representation of the scene.