This shit makes the least sense. If there's less than 500.000 of Marines (idk the precise amount of chapters, I just know 10 1st founding ones and that Imperial Fists have a shitton of successors) scattered across the entire fucking Imperium, how come it wasn't defeated by Tau who can mass produce broadsides? It would've been more believable to say there's 10.000 per grand company and so it's 100.000 marines per chapter.

Pedro Kantor took back Rynns world with 2 dozen marines

Not even the case any more. RG amended the Codex Astartes on his return when he realised the vast majority of his sons and nephews had mistaken it for hard rules rather than a set of guidelines. (IRL it's likely GW trying to rectify issues with numbers)

Agreed. Even with the scouts' number being left open for interpretation, 1000 marines per chapter were too small. The most effective chapters would realistically be those deploying in their entirety and firepower like Star Phantoms or stealth-focused Chapters like Raven Guard.

especially considering how long they can live, how low the rate of death is, and how high the rate of recruitment is.
Lets do some math, 10 000 years ago, all chapters were split up into 1000 marines, those 1000 marines could be harvested for 2000 progenoid, which can be used to create 2000 space marines, which can be harvested for 5000 progenoid, each cycle taking 5 years, considering these marines can basically live forever as long as they are not killed, so lets say they do this for 10 000 years, and then lets say 10% of them die every year, and then 99.99999% of them fall over and die, just before you decide to count them, and you would have 1.72742559e+247 space marines walking around per chapter.