Ironically, certain psychoactive substances like cannabis (high CBD and low THC) have been shown to be effective in the reduction of stress in schizophrenic people and doesn't contribute to psychosis even within families who have members with it. Interestingly, some use THC for their schizophrenia symptoms.
However, environmental factors (set and setting) can greatly increase the likelihood of onset of schizophrenia too.
On tulpamancy, I've heard of some (myself included) who "hear voices" but they're not the result of psychosis, but an internal dialogue and hyperphantasia. I was freaking out needlessly for years thinking I had schizophrenia.
There are some instances where the psychosis is benign, and in Western world psychosis seems to present itself much more negatively. This could indicate that upbringing and other environmental factors could contribute to positive negative experiences.
As someone who has an internal dialogue, hyperphantasia, and synesthesia, this sounds kinda risky to me. I'm all for imaginary friends (I'm 37 and still have mine), but putting this much work into creating a tulpa seems like a waste of time and frankly, kind of dangerous.
I've had times where the "voices in my head" have saved from suicide, but they weren't forced. They just came to me intuitively. If someone is consciously trying to create a "living entity in their mind" there's no telling what they could do with it if they had the mentality of malicious intent.