It's not me going down that route, it's the flat-earthers.
> Probably still you think Scientology and the Book of Mormon are "dumber" than flat earth theory simply because they sound weirder to you
No. You are raising a straw man here. It has nothing to do with dumbness (though I do admit I find some aspects of Mormonism and many aspects of Scientlology to be pretty dumb.) Nearly all religions are "easily" proven false simply by the overwhelming evidence that there is no afterlife, that mind is a function of brain, and when the brain dies, the mind dies with it. That's for a "generic" religion with no specific claims about the nature of the afterlife, whether or not there are deities, etc. etc. Scientology and Mormonism (to cite but two of many noteworthy examples) go on to make many more specific false claims, each one of which makes the theory easier to debunk.
But of course this is not going to convince anyone who adheres to a religion, which is my whole point. There really is no difference. Religion and flat-eartherism both have equal scientific tenability (i.e. zero). People adhere to these beliefs for other reasons. And in both cases, those reasons are not entirely illegitimate.