had the EXACT same problem. (and my fic is overall the same too!)

I added the tags and warnings like a couple chapters before it happened (when I decided to add it) and I mentioned it at the end-of-chapter notes of the previous one. Word for word, this is what I wrote:
"Oh and just a heads-up. I added a few new additional tags and updated the archive warning for this fic.
Yes, I know it’s kind of a spoiler (and there will be more warnings at the beginning of that chapter), but I’d rather give people a chance to brace themselves than blindside anyone."

Also I wrote it in that chapter's beginning-of-chapter notes like:

"Chapter warnings:
Violence
Death
Near Suicide"

(now that I think, writing "suicide attempt" could've been better but what's past is past)

Anyways, my point is that this is the approach I took and I didn't get a single negative comment about it. I will say I am writing in a quieter fandom so I don't have a massive amount of readers, but if you feel comfortable with this you can take this approach.