also I should point out that his timestop has a cooldown and he's only high 8-C when sufficiently angry
Time Stop really doesn't have a cooldown unless we're using Part 6. There's a reason why his sustainability has an E in Part 6 in the MIH arc specifically (it's becomes his ass was in a coma for 4 months).
The cooldown is never mentioned before that point, in fact we're shown the opposite, he uses it in succession, with no obvious delay between them at certain points like against Ratt.
Also his timestop in part 3 is only for a second or two as he only just obtained it,
His time stop in Part 3 is actually the full five seconds, we're told that twice in both the manga itself and guides, Jotaro's time stop was at its best coincidentally right when he got it.
not like part 4 where he spent years training it like dio did
The exact opposite is true here. Jotaro actually stopped bothering with time stop for a whole decade and when Part 4 started he could only stop time for half a second, and by the end he could stop it for 2 seconds. He never hit 5 in Part 4.
Also Dio didn't even spend half a year training it, he only learned it he had a few months prior to the start of Part 3, and he hit a limit of 5 seconds because his body was weakened due to Jonathan rejecting him, don't know why his body being weakened mattered, but it did explicitly.
Edit: Actually funnily enough JoJoveller points out how Part 4's time stop is below Part 3.
"At first glance, he overwhelmed Josuke's Crazy Diamond. His ability to stop time is as short as two seconds compared to his heyday when he was 17 years old."
