There’s no mystery to what is and isn’t. It’s pretty simple, a matte painting is an image used in a live action shot to replace/extend part of the plate itself. Typically used to extend existing sets, replace environments etc… One golden rule; if it looks like a painting then it doesn’t succeed as a matte painting, and so far everything you’re doing looks very painterly.
The image with the poster boards comes pretty close, though there’s a few perspective and scale issues.
Take some video footage and do a set extension, background replacement. Make it look photo realistic, no fantasy or sci-fi.
Drop the edge glows and color filters.