> There's an -ism for that

It's Buddhism.

https://en.wikipedia.org/wiki/Anattā

> Actually, a few different ones depending on the exact angle you look at the it from: solipsism, narcissism,...

That is indeed a problem with it. The Buddhist solution is to make you promise not to do that.

https://en.wikipedia.org/wiki/Bodhicitta

And the (well, a) term for the entire problem is "non-dual awareness".