I want to allow my site's users to submit content for consideration/review. Does a cover photo of a 3:2 ratio fit in well with all screens? Is it what can be considered consistent? What is your best advice/recommendation?
Best aspect ratio is: the one that fits the narrative and composure 99% of the time.
The aspect ratio there is really, really funky. Something like 1504 : 607. If I had to use any other format it just wouldn't be the same image.
Unless there is a technical limitation, you really should leave them be. Instead look for ways to normalize their presentation without changing the aspect ratio. Something like Flickr's black box works well.
This is not how it works. The user doesn't have to think about such things as aspect ratio. Most people will not even know how to do this. Instead, a web site will have a style and as such have space for an image to go into. When it displays an image it allows maximum width and then optionally controls the height to avoid misuse. You don't want to images to break the style of your site by giving user total freedom. However the advantage is, photos are mostly standard sized. They are made by cameras and processed to be displayed on screens. So although dimensions will range from Width or Height being larger, most photos will just work.
If you want to accommodate things like extra wide panoramic shots, resizing those will make them useless, so the site will have to be designed with that in mind. But this is not what the question is, so I won't go into details on that.