Unfortunately you've missed one very important mechanic in the anime.
With each new region, something seems to reset Ash back to 0 as a trainer. Also consider this: Ash is a career trainer. There are many trainers that are arguably way older than him. It's not a coincidence that most of your typical Gym Leaders are not young. Even if we assume that Ash is now technically 28 years old, which he is not, he's maybe 14 or 16 at most, he still has plenty of time.
It seems to me that often Pikachu is the Last Pokemon he's using. I imagine this is due to Pikachu being basically Level 100. I presume that Ash is encouraged to, not only by the Pokemon League, but by the professors as well, to ensure that he does not rely on Pikachu solely. There may well be something additional going on, where they happen to be ah, nerfing, his Pikachu not only to keep it healthy and happy, but to prevent Pikachu from destroying things. It's been long noted that Pikachu has always been uncommonly a strong one for his species. It's even threatened Pikachu's life a time or two, early on in Kanto and a few other times where he gets so sick because of his high power. This may be a unique condition to Ash's Pikachu too.
It seems to me that Ash doesn't really care about becoming a Leader or a Champion, he just wants to be a really good Trainer and travel the world. He's literally "Travelling across the land, Searching Far and wide, for each Pokemon to understand" Secondary also is his mission given to him by Professor Oak, and is subsequently renewed upon arrival in each reason. He's filling his Pokedex. That can take an incredibly long time to do, and that's not even requiring him to CATCH them all, he's just seeing them all. Sure, he WANTS to be a Champion enough to try. But Ash has values he will not compromise on, and this often means he has to forge a more difficult path for himself. There's no shame in that though.
Ash is basically a very unique type of Trainer. At the heart of it yes, he is to follow the path of all trainers and is to conquer the Gyms and the Pokemon League. But there's also another facet to his job. He's aiding research! Very capably too! Think of Ash as a kind of very capable caretaker. He's able to take almost any Pokemon he can actually catch, and turn it into not only a competent battler, he also tends to be more capable than most trainers are in understanding his Pokemon. He understands Pikachu fluently. He can hold a conversation with his long time partner as if it were another person. Not only that, he can develop this capability pretty quickly with other Pokemon that he brings along too, though they never quite get as close as he and Pikachu actually are.
Furthermore it strikes me as not so coincidental that oftentimes Ash's travelling companions for each region almost always happens to contain two types of trainer: the first one being a Gym Leader (Either To Be/In Training, Current or Former) whom he often picks up on the way after picking up the second type, which is always an absolutely FRESH, NEW and GREEN trainer in the region. In essence, Ash after Kanto, where he was fostered appropriately by two Gym Leaders, takes on a very pivotal role in other regions where he also fosters and travels with new and budding trainers seeking to take on the Pokemon League.
It's also no coincidence that Ash picks up a Gym Leader or two. Often Ash in his travels encounters endless troubles in the form of criminals who make the path dangerous for other trainers and people. In the Pokemon World, it's generally accepted that in lieu of an Officer (Jenny) or similar Law Enforcement Officer, it is the duty and job of a Gym Leader to uphold the peace and law. Gym Leaders, being incredibly experienced and talented leaders and trainers are allowed to recruit as many trainers as they need not only to run their Gyms, but to deal with law breakers and Pokemon incidents as necessary. It's not something I find as coincidental that Ash just seems to turn up as a very commonly utilized trainer, given his massive experience and number of Badges. The sheer number of times he deals with Team Rocket, Aqua, Magma and Galaxy shows that as a trainer, he's purely muscle. He has the power to back up anyone actually entitled to enforce the law, and he often has a sense of justice about him often enough that he will do so without hesitation.
It's also well known that Ash's Mother, Delia is actually a very well connected person. Through Professor Oak, as any parent would do, she makes absolutely certain that Ash is provided for, looked after and has good company along with him. It's absolutely necessary, a trainer as gifted as Ash is tends to draw trouble the same way bees are drawn to honey.
Is Ash the "Very Best"? No. He actually has a long way to go. He's too young arguably to even be expected to be that skilled. But consider this, if Pallet Town ever were to expand, and Ash were to come back home and Settle Down, or even decide to settle down with one of the many female travelling companions he has had, he would likely immediately have the mantle of "Gym Leader" thrust upon him should the City he reside in grow enough to warrant a Gym actually existing.
This isn't even considering that the Pokemon Leagues seem to fix upon a single number, 8, as the Maximum Number of Certified Pokemon League Pokemon Gyms in operation at one time. So in a way, Ash is like a Travelling Gym Leader. He is literally a trainer the Pokemon Leagues seem to allow to travel around and sharpen their Gym Leaders with. He may not be the best or the brightest crayon in the box, but he's definitely always placed decently at championships, and I imagine that each League is going to send their say, top 20 to top 50 trainers along to the next Region, so long as they entered and competed. I've yet to see Ash place in less than a Top 20 in a league.
This doesn't even consider the fact that Ash has a positive track record in dealing with "Legendary" pokemon who are generally considered to be forces of nature and not to be captured by trainers. That's the thing though, it's hard to say that Ash ever "Captures" any of his Pokemon. He befriends them first, somehow then captures them. He's definitely an innocent enough kind of person that, Pokemon inherently trust him even if they're super powerful and ridiculously wary of Humans. In many cases where Ash seems to be involved, he does whatever is needed to ensure these "forces of nature" cause as little collateral damage as possible, with full cooperation of the Leaders and Pokemon League and Law enforcement. Because Ash approaches each Pokemon as if it were as sentient as another human, he can communicate and interact with them in ways most traditional trainers can't or won't.
It also seems interesting that despite his battling skills not being top notch. He still seems to be able to progress past each League after being disqualified. I feel as if the League recognizes that Ash is a good example of a Good Trainer and tends to prefer him around as a way to keep trainers and leaders honest.
