How to remove featured image from gallery / slider [ Mono theme]

I am using the Mono theme by Cudazi (https://curtziegler.com/sitedemo/mono/). I want to change my thumbnails because the auto-generated ones don’t look very nice, but if I upload an image and set it as the Featured Image, it automatically shows up in the slider, which I don’t want it to do.

I can’t crop the thumbnails using the built-in WP feature.

I installed a WP plugin that allows me to remove the Featured Image from the gallery, and while this works, this doesn’t affect the slider in the Mono theme. As far as I can tell, though, the Mono theme relies on the gallery to produce the slider.

Is there a way I can edit the code so that it says something like, “If the image is set as the Featured Image, do not show in slider or attachment list?”

Please help!