What can be done to make posts here visible on the main subreddit, so users can be more aware of discussions here?

Perhaps a bot? There is one that notifies mods on Discord when a new post shows up here.

If a new post comes here, perhaps a bot will cross-post it to /r/UFOs, but locked and read only, directing users to come here?

The post on /r/UFOs would be a self-post, with a link to this exact discussion, and then a duplicate of the text body post here, if any.

I'd recommend it run on a delay of at least one hour, to give the OP user here on /r/ufosmeta to edit and tweak their post briefly before the cross post happens.