I’ve followed all the frequent posters on piefed.social using my mastodon.social account so we should be seeing a lot more PieFed content showing up there now. But unless you use hashtags, I’ll be the only one seeing it!
When making your post, put a few words into the Tags field below the Body field, separated by a comma. No need to include the # character.
Mbin already does this afaik (the default tags, not the prefixes), so I don’t think that would be controversial.
Lemmy does it as well, usually posts receive a tag corresponding to the community name
True, but Mbin lets you define a list of tags for communities to apply to posts. So it’s closer to OP’s idea of “a list of default tags”.