Fascinating. I don’t remember saying Descartes is Assur.
Pashut Pshat in Descartes is that there is no existence. Nothing exists. The only things that we can know to be true are our own thoughts. We cannot ascertain the reality of anything else. Hence, the only reason and only place anyone exists is in their own thoughts. That’s the only place existence can exist.
Which, ironically enough, is very close to what yekke2 said. (And not at all what the Rambam held. CH”V to pin pantheism or panentheism on the Rambam.)