(Sorry for the long post in advance)

I have a few questions about Spinoza’s substance monism, which I’m quite new to. Am I right in saying this is the (broad) outline of his argument:

1. We first take Descartes ideas of substance, attributes, and modes. These things are defined slightly differently under Spinoza, for example attributes are no longer the essence of a substance but what the intellect perceives of a substance as constituting its essence. Furthermore, unlike Descartes, Spinoza does not allow for causal relations between attributes (they are entirely parallel and independent to one another), instead arguing that causal relations must be explained, and are therefore basically just conceptual relations.
2. He then goes on to posit that two substances cannot share all attributes, as given the transitivity of identity, and the fact that all modes are explained through attributes, they would just be the same substance if they had the same attributes.
3. We now know substances cannot share all attributes, but what if they share some? For example, substance 1 might have attributes A and B, substance 2 might have B and C. If this were possible, through attribute B we would be able to conceive of both substances, which would go against the very definition of substance in something that is only conceivable through itself.
4. So, no two substances can have any attributes in common.
5. He then goes on to explain how the essential nature of a substance is to exist. Given the principle of sufficient reason (PSR), a self-caused thing like substance must have a reason for not existing, if it indeed does not exist. This reason cannot be external, however, as the very definition of substance is that it is entirely self-sufficient. Hence, any reason for non-existence must be internal. If a substance is internally coherent, it must exist according to the PSR, as there would be no reason for non-existence.
6. So, he basicallyapplies this entire framework to God (a substance of infinite attributes), and concludes it exists. Given (4), we then know that no other substance can exist alongside him, else there would be two substances with common attributes.
7. HOWEVER, if we apply this framework to another substance (lets say substance 1 with the attribute of thought), we could then prove that that substance exists instead, which would preclude the possibility of God existing. Spinoza would counter this by applying the PSR, essentially saying that there is no valid reason for the non-existence of all the other attributes (eg extension) in this substance. Thus, God is the only coherent substance that can exist.

I have further questions:

1. Spinoza’s argument as to why God is infinite is basically just via applying the PSR, and then arguing that there are no valid reasons for the non-existence of other attributes (I think), therefore God must have all attributes (which apparently, is an infinite number of them? I still don’t really buy this). What if a substance is self-limiting? What if a substance cannot have attribute Z, because it itself says so? Why can’t a substance be finite out of its own nature?
2. Isn’t the PSR alone enough to argue for a single, self-causing being (substance)? For example: The PSR needs some necessary, self-causing being(s) to avoid an infinite chain of regress→ There could be more than one necessary being, but then we would (applying the PSR again) necessitate the need for an explanation as to why there is more than one substance, meaning there would be an explanation of sorts prior to the substances, rendering them not substances → Therefore there must be one self-causing being.
3. Within academic philosophy generally, how are these arguments and Spinoza as a whole typically viewed?
4. What are the main counterarguments directed toward? I’ve heard that most people who disagree with Spinoza do so because of Kant. Does this mean the main counterarguments are directed towards the PSR?
5. As I understand them, attributes are the fundamental ways or lenses by which one can view the world? For example, we can view the world as entirely physical or entirely mental (without reference to the other, hence the parallelism?). Is this correct? Have any other philosophers posited other potential attributes beyond these two?