GOOD POST. VERY, VERY GOOD POST. 100% AGREE WITH ALL THIS.
I have very strong opinions about tagging and systems that implement them myself. Although in my case it's more about filtering than discoverability, but both are very important! Itaku, at least in theory, also initially set out to address this issue. But I've always felt like booru-style sites had more or less the best idea. e621 in particular is pretty rigorous, though I feel like no place has it 100% right yet.
My ideal would be for a site to have a system like e621 for tagging, with a "suggested tags" feature that others can use that distinguishes them from the ones that the artist used. Then either the artist or a mod can confirm them. Ultimately, mods should have final say, of course. And then for filtering, the way Cohost and a few other sites do it, you can even have a multi-tier set up, or different toggleable filters like Mastodon, where in some cases, you just spoiler things that get hit, so you can still view them if you want. (Say, it contains something you're not fond of, but might tolerate if the other tags suggest it might actually still be overall relevant to your interests.) And then there are the "absolutely not" tags, which, when a post has them, they effectively vanish from the site from your perspective. Ideally, the latter shouldn't just rely on tags though and search entire posts, but I digress.
Tags are critical, people. We should all push to make systems that use them as robust and powerful as possible. And consistency is important for making such things work. Not to mention, as the OP stated, implications are an incredibly helpful tool as well to make everyone's part that much easier.
Just... yes to all of this.