So just to boil this down to drop dead simple… OP’s question was what to fill in to three settings to configure:
(1) BUCKET NAME – so it is being said that dots are not… recommended? not allowed? I suspect this may not be a problem for OP. (He just needs to separately figure out a way for his CDN to get images to cache and serve.) So we’re all on the same page here, right?
(2) S3 ENDPOINT – leave blank, nothing needed if he is using AWS, otherwise he can fill in for another provider?
(3) S3 CDN URL – is this simply the base URL that Discourse will prepend to the image path? If so, then that is simple and straightforward and the OP can configure his CDN and supply that base URL here.
I don’t see where SSL wildcard certificates factor into this. OP was told a dot in Discourse config is bad because it will break his cert. But if he is using a CDN or cache then bucket name could very well be irrelevant to the cert, right? If it will break Discourse in another way that is useful to know.