I understand the idea that these reforms were a single set of principles that a single person wrote down at a specific point in time is not the way many historians think it worked. But to the degree which we know anything about specific specific military reforms, how would they be:
Conceived of - was there academic study of military doctrine to draw on? Would somebody in the field simply have a bright idea? Would they bring it up in a meeting? Were there brainstorming sessions? How were specific proposals made and by whom?
Designed - how would it go from bright idea to a more specific plan? Would you delegate to a subordinate to go put some ideas about maniple structure down "on paper" which you would review? Would a commander spend a lot of time figuring it out themselves? Would consuls or other senior leaders be elected on the back of specific reforming policy proposals?
Implemented - Would you wait until the formation of the next army to put your new ideas in to action, or would you go out one day and tell the men you wanted them to divide up into young, middle aged and old, and try something out?
These are all probably quite unrealistic examples, but I'm specifically interested in the generation and design of the proposals to the extent we know anything about it.
Conceived of
They weren't
Designed
They weren't
Implemented
They weren't
There were no "Marian" reforms. Moreover, that term means different things to people who read pop history as opposed to historians. If you ask the internet the "Marian" reforms were a change in the structural organization of the legions and in their equipment. If you ask the historians who came up with the term (they're all dead) and who still use the term (they're almost all dead, with like two or maybe three exceptions) it refers to the abolition of the property requirement in the army and the subsequent change into a purely volunteer force made up mainly of the urban poor. There's no such thing. Here's another person saying that it didn't happen. The Roman army continued to recruit mainly through conscription for the total period of its existence. There is no evidence that the land requirement was ever formally abolished, but even in Marius' time it pretty clearly wasn't upheld like ever. The overwhelming majority of recruits came from the rural regions, especially those settled by "martial" Italian peoples like the Marsians, who were eventually replaced by other "martial" provincial peoples like northern Gauls who were conscripted instead when Tiberius formally abolished the Italian levy.
What about the equipment, and the organization? Military historians like, or at least liked, to talk about a systematic Roman army that had fairly standardized equipment and regulations. More and more it's becoming clear that this is a thing of the Principate, possibly as late as the second century AD, and even then it never really existed in the way popularly imagined. What we do know about the army's equipment and organization in the Republic is essentially two data points. Polybius describes the army of the late third century or the early second century (it's a bit hard for us to tell), and Livy's description of the same army, though written much later, corresponds generally with Polybius'. Our next really clear description is Caesar's a century later. And Caesar isn't interested in describing the army, it's just that he provides enough incidental detail for us to be able to pick some things out. Everything else is later or is not sufficient enough for us to learn much that we wouldn't learn from one of those two. In those 150-100 years some stuff happened in equipment and battlefield organization. The Romans stopped--generally--providing their own cavalry and skirmishers. Military tribunes definitively stopped being upper-level commanders. Cohorts already appear in the narrative in the Hannibalic war, their use was probably not new to the first century. The infantry classes of the front, second, and third ranks were not abolished, but the distinctions between their equipment--which may or may not have still existed during Polybius' time--ceased to exist. Little of this was ascribed to Marius by the Roman tradition, but was added to his portfolio by modern scholars of the nineteenth and earlier twentieth centuries, as well as the incorrect reading of Marius' recruitment in the Jugurthine war that I mentioned in the links from before. Later tradition--but no one contemporary to the Republic--ascribed to Marius the introduction of a new kind of javelin (but Plutarch's description of this javelin is pretty much the same as the description of the javelins in Polybius several centuries earlier and both contradict Caesar's description), the habit of marching light (the Marian mules thing is incorrectly assumed by the internet to apply to all soldiers, but Plutarch is quite clear that travelling without a baggage train was unusual to Marius alone), and a few other minor things.
That all happened over the course of almost two centuries, since we can't be sure that much of it was in place before Caesar at the earliest. Some stuff, like the use of cohorts and legates in battle, appears in the accounts of actions long before Marius. Other things commonly attributed to Marius in the popular imagination, like lengths of service of fifteen or twenty-five years or whatever, we can't really attribute to Marius. There's no evidence, for example, that there was a set term of military service until Augustus, and Caesar seems to contradict this outright. I'm not even sure we should be talking about terms of service in the Republican army at all, because even in Caesar's time it was essentially still a militia, just one that was expected to be in the field for a couple years at a time or more. Caesar's troops expect to be released in 49 not because they have a fixed term of service that's up, but because they've been under arms for ten years and want to go back home. They expect the army to disband and go home after its campaign, and when it doesn't they're pretty upset.
I don't know what the "Polybian" reforms would be, I've never heard that term used. Polybius didn't reform anything. I've never heard it suggested that there was some singular groundbreaking event that resulted in the army that Polybius describes. It's pretty clear that the army that Polybius describes was an army that had developed over the course of several centuries.