I don't agree, but I understand what you're trying to say. In fact, what I got from the show was that OVERCOMING that power was what he was capable of doing.
He clearly progresses from runt who could take on a few people, mostly because of his devil fruit power, to someone who is a capable fighter with strength that rivals really great fighters. He does this because of the reasons you listed.
It doesn't even make sense that he would overcome "power levels" by just believing. They are more powerful. If someone can punch you hard enough to blast your molecules to the winds, you lose, period, it doesn't matter if you're strong willed. Like I said though, because of that strong will and determination, we watch him grow from arc to arc. This is no different than Zoro and how we see him start to train every time he has free time. He's growing, he's determined, he's overcoming levels of power that others will never obtain, all because he has the will to do so, just like Luffy.
Also I mean, Luffy's power was never not that he has a decent Devil Fruit. The amount of times he lives or is saved because he's a "rubber man" is way too high. If he has just about any other power there are a lot of times where he'd be dead. I get that it's a theme in the show, but I don't know where anyone gets the idea that their determination is the actual physical reason they continue to succeed, instead of the driving force that allows them to obtain what's needed in order to succeed.
"Hahahah! Your power level is way below mine! No one knows or cares about you! You can't even lay a finger on me!" -> cue Luffy running through a wall of fire just to deliver a punch to their face
This was because he was a nobody. Even after his first bounty, if you took his 30m bounty as his power level, you would be dead fucking wrong, and risk dying if you fought him on that assumption. Similar to if you fought Zoro or Sanji, even though they didn't even have a bounty. They overcame these things because they actually WERE That powerful, not because they didn't care about the difference in power. If that was the case Zoro would have cut down Hawkeye or something. That's unreasonable. That level of power was FAR away from their abilities, and it took them quite a long time to get close to it. By the time they did they learned a lot about what they were lacking, and what they had no clue about, such as Haki. To clarify, he was always comparable to other people easily, the issue was that he was brand new on the scene. He didn't even get his first bounty until he was already walking tall pushing into the grand line, and his crew mates who were easily on his level didn't even have bounties yet... They took out people in East Blue with ease who had upwards of 20m bounties that they had probably had for years. You could compare them all back then easily too.
As soon as they were well known and had reasonably up to date bounties, and were "players" in the main game, which they weren't for literally 100s of episodes, you were then capable of comparing them like you are now. It wasn't a lack of their ability, not in the slightest, that wouldn't even make sense. It was a lack of information, not only on the part of everyone on the show, who continually underestimated the Straw Hats, but also on the part of the viewer, who had no idea how close or far away in power and ability some of these character were. When you meet someone like Crocodile you assume he's some kind of monster like Hawkeye, because he's part of the same "group" of people. You learn that this isn't even remotely true, and it doesn't have to do with power in the way you once thought.
As for Luffy winning and all of that, I personally find it pretty hard to think of a time where he wasn't clearly going to win in the end. I mean not only is he the main character, but his tiny crew is still made up of some of the best there is in their fields, along with there being at least 3 of them that are complete powerhouses. The only way you think they are going to actually lose is if you don't realize they are clearly far stronger than other people believe they are on the show. And that's not hard to do as you're basically shown this every single arc.
just felt calculated, predictable, spelled out.
And then you literally go into detail on how fights worked even back in east blue in the beginning of the series. Literally "GOMU GOMU ROCKET!" Oh no, Luffy got hit through a wall. WOAH he's gonna do BAZOOKA!!! and look the guy is flying away. It's a Shonen, I don't know where you got the idea that it would be anything different than this. When you look at it without your nostalgia goggles on, all of these shows play out exactly like that.
And the whole time you knew Luffy was going to win.
This was never actually in question, as he's the main character of the show, and every single main opponent he has is obviously going to go down. In great shonen sport, he's obviously gotta get beat up enough to get mad first though, like every single one of these shows. That's not new.
If I'm completely honest, I think you've just lost site of what the show is and has always been. You seem to not understand that the show is ~700 episodes long. He's done the same thing at the base level arc after arc. It's a shonen, there just is not any way you can get around that. I don't know where you watched that you thought there was any idea that Luffy was gonna lose, but the only times he loses is to farther the plot a bit, but then he comes right back and wins. This is literally all Shonen. There was never any "risk" in this regard.
As for the idea of Haki itself, the only reason I don't like it is because I dislike the idea of people having basically powers. It feels very much like it could develop into a much more focused style ability range, and that's just, really odd. As someone said in a comment to me a little bit ago, the idea of DBZ style blast seems awful, but the progression of an idea like Haki seems open to it, which is really out of place in my mind. It's an extremely long show though, so there had to be some progression of power that limits people progressing.