Your account of Plotinus is similar to the view of Hegel. I’m more familiar with Hegel than Plotinus, but your basic statement that the problem is to reconcile time/change with eternals? The Hegelian solution similar to that. I.e. Eternity substantiates itself through the process of time, etc. Hence both universals and change/time are given their due.
The real problem isn’t how to reconcile eternity and time or being and becoming, but rather the real problem concerns what the status of being itself.
Being, nature, essence, eternals are often used interchangeably by philosophers. They aim at the same thing but do have different specific meanings in specific uses.
For Platonists the truth of a thing or an entity is that which exists at all times for it, and is not a mere accidental property of it. Hence this definition needs eternity to be a possibility.
Likewise the definition of nature is what exists beyond convention, beyond what our mere opinions of it are. And why the dialectical method is the only best method to to establish it. Since different people speaking on the same subject will show difference, conflict of our projections and interpretations, and the truth remains when you remove what contradicts.
If knowledge is possible, it is knowledge of essences, the permanent parts of entities, and each of these entities must partake in a logical unity / intelligibility. There must be forms and classes that clarify and categorise. Hence why Socrates/Plato was correct to infer that if intelligibility is to exist at all, then there must be universals, that the inner logic of logic points to unity and eternity of all.
The meaning of parts depends on greater parts, and these parts must in turn depend on a greater whole, and this whole must be intelligible for any of the parts to knowable as such.
But does Plato ever show that Socrates has reached understanding of the whole? Most dialogues show that he reaches an inconclusive conclusion, a paradox, or aporia. It seems that the whole is elusive. Which would imply that because the whole is not knowable that the parts are neither knowable at all, as we can not grasp the essence of any object properly.
It is not even clear that there is a ‘whole’ at all. That existence isn’t just a collection of heterogenous parts with no underlying unity. Knowledge / logic implies unity, or a common basis to beings, i.e. it implies a unity of being. But implying doesn’t mean it’s true.
For knowledge to exist at all there has to be some sort of permanent horizon of meaning that gives credence to the human ability to categorise by type, its ability to order nature.
It would seem that we don’t have the ability to know the ‘essence’ of anything. But maybe we can still have knowledge of things through dialectical reasoning. That is we can see that some things exist beyond our use/labelling/opinion of them. That is, through Socratic investigation we realise that forms/universals exist in a pragmatic and inevitable sense. That they are a necessary feature of our reality since we are ‘rational’ and ‘logical’ beings, i.e. we cannot but help see the world through language (ideology as a Marxist would put it), we can’t help but universalise and categorise on a basic understanding of a unity of being. Universals are in performative in a way that the Big Other in Lacanian psychoanalysis is. A fake but fundamental part of reality, for us.
The reason why sensing is bad (and sensors inferior!) is not because they see the world as heterogeneous, in constant flux, as it is, but exactly because they see the world through universals, genus, species, type, ideology and are non-reflexive about it—this is why the Socratic turn to investigating ones assumptions/logical meaning is important.
The Socratic turn from investigating how things appear to us to investigating how we understand concepts in the very first place, hence an inevitably politically tied investigation.
And so Socratic philosophy can be seen as attempt to understand the parts of the whole, to classify each thing as it is itself, what it is.
It fundamentally removes meaning from things. What’s most interesting is what cannot be understood, because things that can be understood have conventional meanings—they have ends that are bordered / bracketed off from the whole.
But Platonism is redeemed, as universals / ideas on the whole are sorts of ‘regulative ideals’ in the Kantian sense, they point to the truth of the whole, that there’s NOT a definite horizon, but there’s a truth beyond convention, all is not relative. And this elusiveness of the whole does not provide universal truth to reality as such, but grounds a permanent truth about man’s position in the whole, which is enough to grant legitimacy to a knowledge of kinds, essential knowledge about man, what is his purpose, and hence legitimises the connection between the true and the good that defines Socratism and redeems the Platonic ideas as guides to what is true and what is good etc. etc. etc. etc. etc.