I wonder if tags could be treated here as side information instead of always being provided and shown inline (like image descriptions)

The mastodon internal format already stores tags as structured fields, so limitation seems to be in the status posting function having no separate tags field.

Another option could be a convention in apps not to show tags at the end of a post inline, but hidden by default, or in their own list?

#ThisHashtagWouldBeHidden
#MastodonAPI
#FollowingHashtags