I promise one day I’ll dive into this Platonism you keep referencing, but for now I’ll sidestep it to mention that I have indeed gone crazy over the question of why anything exists at all. It can be haunting or exhilarating depending on your mood. But if I read you correctly, if nature is a manifestation of God and we are a part of nature, that would make us a part of God, right? This reminds me of Jordan Peterson’s rough definition of God as “transcendent reality.” Insomuch as we are entirely beholden to this reality which has produced us all on its own, I can understand deifying it, but that at best gets us to pantheism, which I’m sympathetic to on particularly starry nights. I just don’t think this qualifies me to say “I believe in God” any more than owning a toy car qualifies me to say “I own a Lamborghini.”
Are we part of God is a very good question, and you're going to get different answers there depending on who you ask. For myself, I would maintain the distinction between Uncreated and Created, but there's a lot of extra stuff underlying my position on that. The most important factor is my philosophy on time--I think special relativity and what it says about time as a property of the universe vindicates the medieval picture of God as apart from Creation, viewing past, present, and future simultaneously, and yet from within the universe we obviously experience things differently, so I do see a strong disconnect there between God and the universe. Enough that while I would say that God is maintaining it in existence, the universe is not itself a part of God.
Whether or not we are ourselves a part of God is an entirely different question, though. As far as I'm concerned, that depends on what's really going on with the phenomenon of consciousness, because while I do think the mind is material, there seems to be a strongly transcendent aspect to it. If you're not going to be a dualist about it and claim that God has individually granted each person a soul, then you're effectively stuck at something closer to a world-soul concept, so... yeah. In some sense I would consider us a part of God, or at the very least that we're participating in his nature simply by consciously existing. (This is unlikely to make much sense unless you drop materialism like it's on fire and run away to India, though.)
But yes, you sound a bit close to pantheism right now. I don't think the border between atheism and pantheism is that clear--I spent most of my college years dancing across that particular line, and I would agree that pantheism does not really entail belief in God. At least it didn't for me. You need to hit panentheism for that title to fit in any meaningful way. If you end up having to attribute will and awareness to the transcendent reality (and there are genuine reasons to do so), then you are at God.
And concerning going crazy over the question of why anything exists at all, welcome to the wonderful world of existential angst.

I've actually seen it suggested that this is the real issue that the ontological argument is aimed at--for a theist, the next question is why there is God instead of nothing, and what it would mean that he exists by his own nature. I get nervous about this level of intellectualizing about the unknown, but it does make for an interesting thought experiment.