This mod let's you remove individual vanilla simulations from the main menu background.
There is a similar mod by LegitimateTeddy. Check it out for some custom backgrounds. LegitimateTeddy's, DaveMcW's (thumbnail) and my mod work without interfering with one another.

Credit for some parts of the code: Honktown
Credit for the simulation used in the thumbnail: Main Menu Micro Factory by DaveMcW (Prolly my favorite simulation)
Thanks to the guys on the Factorio discord for the help.