I think the main issue is that you're not taking into account that in order for him to live in todays One Piece world, he would have to know Haki and how to fight. He's only the way he is when we meet him because Haki didn't exist, and I don't think Oda really knew who and what levels of power he was going to introduce.
He introduces the DF abilities of Enel as nearly on par with the level of display we see from the admiral Logia users. Maybe not exactly, but he created a storm while fighting, and has absolutely enormous attacks and power displays, including range that prove that he's trained and gained abilities that would put him out there near those users.
I think even within the world it makes perfect sense that he wouldn't be beating admirals or fighting yankos, but he would be close, a lot closer them you seem to think.
The main issue here is simple that it's not a simple thing to place him, as a lot of ideas about fighting and what people do while fighting are completely different than they were back then. It's unreasonable today to think he could get to that level of power and not be able to take hits from Luffy back then or fight at a level that would allow him to perform actual martial arts combat in a way that would have allowed him to overcome a weakness that didn't allow him to utilize his DF fully. This is exactly the same thing that we have with Crocodile, who has in more recent times shown that he was much stronger than we were shown back when he was introduced. As a Shichibukai, he would have ABSOLUTELY HAD TO have known how to utilize all of the haki, and would have to be a competent fighter even without using just his powers.
We are all just guessing, so I'm not saying I'm definitely right or anything, I'm just simply trying to get a point across about the level of ability he showed, and how, if he was brought into the story now, a whole lot about his character would have to be completely different, and it would be unreasonable for him to have the weaknesses he did back then, which as I said, were due to the lack of world building on Oda's part.
The only reason I bring up the idea of the 500m bounty is not because it's pure power level, I'm not saying that, that's not the point, but the point is that he wouldn't get that kind of bounty, and would be taken out well before he got to that level of bounty if he could be captured or killed by anyone other than really the admirals.
I think he was far stronger than Ace was, and I don't even think Ace would simply be taken out by the admirals. He was nearly dead when he was on his way over to marineford, and by the time he enters any combat there is no way he's not seriously almost dead already, and he still doesn't just go down. Someone like Ace had the bounty he has because he wasn't someone anyone, even the admirals, could easily catch and take down, or they would have.
His bounty shows that he would be a player on the big field, and in order to do that we would have to retcon some things about his character given the ideas that were not even in place when he was introduced.
I personally think he's be under the yonko and the admirals, just because I think those people are supposed to be the strongest, but I don't think anyone is simply taking him out, and I think he could hold his own well enough given the power display we saw from him.