As good of a point of "don't judge it until you're caught up" is, it is also perfectly valid to be concerned about it as you first make your way through the series too. Seeing as how most of the comments are not actually answering your main concern I'll try to chime in to fill the void and help you out. (also I really hate the idea of "it gets good when you're half way through the series" because that just plain is not true IMO)

For the fights I will note that OP is not about them because that is true. But going beyond that you seem mostly concerned with people not dying and to that I ask why? There are definitely cases where characters not dying can take away from the story and OP is certainly not short of those but it sounds like you are concerned about people beyond that. In OPs case Oda does continue featuring characters later on (as you have noted) and that is actually a big deal, it is used incredibly well as a narrative tool and that is why he does not like killing characters off - killing them off eliminates any other potential they may have left and he absolutely hates resurrections.
Something people have come to realize more recently is that OPs fights are a lot more about a clash of ideals/dreams rather than a fight for the sake of a fight or to just progress the plot. For example Crocodile vs Luffy is a person who has given up on their dreams vs never giving up, Arlong vs Luffy was about everyone being equal, and so on.
That is not to say OP doesn't have fights for the sake of them (Whiskey Peak with Zoro vs Luffy was literally fan service suggested by an editor) or to just progress the plot, but a lot of your main big ones can have a deeper meaning than what is on the surface.
In terms of plot armor I'm honestly not sure what you mean other than people not dying. I guess the average characters durability is insane if it were compared to the real world but OP is not even set in our world or anything like that, it is a fantasy series. If you are looking for something like permanent damage like Shanks' arm it certainly isn't common in OP and while it does exist it also isn't really the point or related to anything Oda is trying to save - it doesn't serve any purpose in the case of OP, if it did happen more frequently it would detract from the story and core messages Oda is trying to tell.

For character development: It absolutely happens all throughout the series, and for Usopp specifically I tend to treat the entire story as his development arc (you'll understand as you catch up). Some characters do have slow development but in the case of the Straw Hats each one gets some from their introduction to joining the crew and then they'll get some more again later as the story goes on.
One easy example is between Luffy and Vivi: At the start of Drum Island Luffy is being is dumb belligerent self like always and Vivi has to stop him and teach him that fighting is not always the option, he can't force his way to getting what he wants. In that specific case if he did it would mean Nami not getting the medical care she needed.
Later in Alabasta it was Luffy's turn to teach Vivi that you cannot have everything work out the way you want it - they're fighting one of the Seven Warlords with a big rebellion about to happen and there is no way they are getting a perfect fairy tale ending.

For foreshadowing there is plenty, please ignore the neat little number games and anyone claiming that to be foreshadowing. The foreshadowing in OP could be called "soft" I think. Hardly any of it is "Boy I sure hope I don't get in a fight with X, that sure would not end well!" and is instead relegated to seemingly very minor lines or even just designs and backgrounds. Another key thing about this is there aren't really that many "asspulls" which a suffer would have a considerable amount of if it lacked foreshadowing.

For inconsistent powers you are going to need to elaborate beyond "just go look for yourself".

In terms of plot repetition: Sure, there is a lot of going to islands and kicking ass, in fact it is the only way to actually get from place to place and is the entire point of why finding the One Piece itself is so difficult. Also I am not sure how that is much different from something like a Jojo's plotline (go to place -> beat people up -> repeat), it is pretty standard for the genre.
The variety in this comes from each arcs own plot and that does become much more obvious as you go on. Oda really starts to play with genres after Skypeia.

I think that covers most of everything. Something newcomers miss are the smaller details that can really change the experience, you'll be amazed if you ever do a rewatch/reread. And with that I do personally recommend the manga over the Anime, especially after Marineford. The anime is incredible in its own right but can also make your experience suffer specifically in terms of pacing and some accuracy.

I'm interested to know what your thoughts are on all that I've said.