I can't describe it, but I know it when I see it

My oldest zygote is 5 years old. Up until now, I've been able to keep a good lid on what he's allowed to watch. We have Roku with a handful of streaming services, and he's comfortable navigating it himself to watch shows. I'm big on teaching agency and responsibility, and he's responded well to that: he watches shows we've okayed, asks about other shows he's interested in, and respects us if we tell him no. He's a smart kid and I'd like to keep him that way.

It's starting to get harder. Originally I could say on on grounds of age appropriateness, or I could point to things about a show that I outright didn't approve of for him. This is important to me because I want to teach him to think about these things and make good choices for himself in the future. So instead of telling him "No, you can't watch that", I want to tell him "No, you can't watch this because _________."

Now that he's getting older, he's asking more and more to watch things. And in some cases, I find myself saying "No. It's hard to explain why, but this isn't good." I hate saying that because I know that's not satisfying at his age, and it doesn't teach him anything about how to make good choices about what to watch.

YouTube is rife with examples. If my son is watching a branded show about Spiderman, I know what to expect. There are brand standards that I don't always agree with, but I know what the damage is, and the problems I have with those shows are problems I can talk through with him.

But when YouTube recommends videos that are low production quality videos of people in their living rooms playing with Spiderman figures, I don't want him watching it. Some of it feels Elsagate-adjacent (https://en.wikipedia.org/wiki/Elsagate) in that it seems harmless but you are just waiting for it to veer into weird or inappropriate territory. There's no trust between me and the content creator. Sometimes they're just harmlessly dumb. But in any case it's hard to explain why I don't want him to watch something that appears harmless to him.

Does anyone have similar experiences? Advice? Commiseration? Perspectives?