My initial answer (which I stand by) is to not include a trigger warning in a query. Let the chips fall where they may.
Here's why I believe that: putting trigger warnings on a book is someone else telling you what you should/should not read. I am unalterably opposed to that. (It's not censorship because censorship is the government making that choice. )It is dangerous because the foundation of democracy is the freedom to read what we choose, unfettered.
Now, am I in favor of small children reading The Lottery by Shirley Jackson? No I am not, but we're not talking about small children, or children of any size or stripe here.
Trigger warnings are intended for grown ups and soon to be grown ups. The implied warning is "don't read this, it will upset you."
I think you should be darn careful about who you give that kind of power to.
Do you give it to me? Not on your life. I'm not responsible for your mental health (and you're not responsible for mine.)
Do you give it to librarians or a form in the copyright office? No no no. There's no way one designation will work for the myriad of needs people have. Trigger warnings are not quantifiable.
So who do you listen to?
People you know, and people who care about you. In other words, your community.
Here's an example of what I mean: Manchester By The Sea is brilliant (if you disagree, totally fine, but this blog post isn't about that, ok?) and deeply disturbing movie. I will never forget it, but I never want to see it again.
And when I was talking to a good friend who has two little girls, I told her to never let her husband see that movie. I was afraid it would disturb him deeply.
I knew her husband, I'd seen the movie. I offered an opinion. He's free to make his own choices.
But once we start labeling things, or giving blanket warnings "this is disturbing" we've removed the "I know her husband" element of the equation.
If you're querying a troublesome book, remember you DON'T know me. For all our conviviality on the blog, my personal life and my mental health are not things you know about.
And I don't know about yours (with some very few exceptions.)
Thus I'm not going to warn you off of Manchester By The Sea other than by saying that movie will haunt me forever.
I'm not going to warn you off The Lottery by Shirley Jackson other than to say it was so disturbing I haven't forgotten it for more years than I care to number, and I never read anything else she wrote.
I'm never going to tell you to not read a Vince Kohler mystery despite the fact I never forgave him for a scene in one of his books, and he's been dead for 20+ years.
You are in charge of your own mental health. If you're finding a lot of books disturb you, you might want to read more reviews ahead of time. If you find a book had a troublesome scene for you, you might want to post that on an Amazon or GoodReads review as your opinion.
Trigger warnings are well-intentioned but we all know what paves the road to hell.