You're ignoring that Ace only lost against blackbeard because his powers were taken, and you're completely ignoring that his vivre card was almost completely gone WELL BEFORE he was actually killed.
We don't really know how strong Ace was on an overall level, but from what we did see, Enel's influence of his area, attack size and strength seem far more on par with the admirals and their influence of their environment than what we saw from Ace.
Maybe, just maybe Ace was actually out there facing off against the admirals when he needed to, but we don't know if he was that strong, at least, I don't remember seeing anything to that effect. Enel was shown to do way more incredible and devastating things than Ace did though.
Bounty is absolutely not a power level, but it's an indication of where they stand on the playing field. In order to have a bounty 500+ he would have to be a world threat, someone they can't simply reign in, and someone who they would have a difficult time defeating/capturing.
Now I'm not sitting here saying "Oh yeah, he would destroy admirals!!!" but I am saying that if he was placed with that level of power, in a period of One Piece where the power levels were actually established like they are now, and Haki existed, he would absolutely be able to fight with an admiral, without a doubt. His range of influence and size/power of his attacks alone are enough to show that his DF abilities are close to what we saw from the admirals when they fought eachother, and the fact that his lightning abilities means he's FAR faster than other people mean that he would be a perfectly capable player at that level.
I know it makes sense to try and point out specific failures in his defeat, but the only reason he lost was because the plot said he had to, and Oda didn't know he would end up introducing and displaying the powers levels and abilities that he did later on. Oda even said he regretted putting Crocodile and Enel that early on.
I really don't see how you think he would lose to an admiral easy. Maybe we could claim that his fighting style and focus was trash, so that's why, but that's simply because he lived as god on a sky island where he had no actual reason to fight. If he was a pirate with a 500m bounty, I really don't see how his level of DF usage wouldn't clearly put his power level up close to the top. I think below it, but sure as shit not "easily beaten."