That's indeed a difficult question. In general, I would assume it is important to follow the guidelines for this for the country the platform you publish on is part of. So in this case it would be the US. That being said, not even that is enough strictly speaking, as recently Steam had to adjust their rating and store page display for countries who "asked" them to follow their guidelines and make a rating based on their age systems.
It basically comes down to every country having different points where they draw a line, as well as there being a reason why you usually have to go through a given process from their side, as they know what to look for and how to judge it.
So the reality here is simply, itch does not provide nor enforce such a system for you to set up a age recommendation, not for the US and especially not for other countries in the world. Which means you can only try to comply with their guidelines and how they define adult content and go by your own impression of how your game would be categorized compared to other titles.
I guess in that regard, asking other users here or looking at other projects on the platform is a good idea. But at the end of the day only you know your game best. Going by your description, it is difficult to judge it entirely. And then again, we have the difference between countries. For example, I would say that most of what you describe would go for "16+" in my country, but I am no expert either, of course. Just going by games I remember playing in the past and what they offered in terms of topics and visuals. But the line can be very thin there.
I think topics that easily put you into adult territory are excessive and unreasonable violence, shown either direct or indirect. The same goes for erotic content. If it is only mentioned or doing the good old fade to black with two short noises, it will not be the same as if it is the focus of the game and shown directly. Nudity is of course something that is usual not allowed in underage media.
The biggest issue is how far you can go with topics that are dealt with on a more mental side, like suicide or abuse. That is definitely content you can see in non-adult games, but it is either only loosely framed or less blatantly put into the idea of the game. I think the more it becomes a focus, the more you may be better off to put your game into the adult category. Maybe not even because it would be mandatory, but because I am not sure if that are really topics a minor should engage with. They have more than enough time to think about that stuff when they are grown up.