No it's not there is a notable decline, art has gotten worse, main cast has gotten worse, and all fights are rushed and off screened
No, not really.
- The art of One Piece just evolved, it has not gotten worse. While Oda did had problem in recent years for some details because of his health problem, in reality, the art of post time skip is much more organic and natural that it used to be pre time skip. Its not as epurated and Oda took a lot more liberties in the compositions and paneling.
- Main cast has not gotten worse, this is also an illusion. Main cast has in fact improved. The only difference is that the main cast is now composed of other strawhats just as it was the case for Vivi during Alabasta. And each time, the cast and the characterization is on point. And the crew is similar. While it doesn't have the same evolution as pre time skip (because let's not forget that each cast had one arc more or less dedicated to them), the cast is still relevant and as still interesting as it was pre time skip. The arc of Sanji on whole cake is a bright example of this.
- And no, the fight are no rushed or off screen. Its just that some fight are not necessary to make the story move forward. One Piece is about the story and not the fights. If you want to see a fight driven manga, you can check MHA for example.
The strawhats have been sacrificed for the sake of side characters and Luffy
No they are not. During pretime skip the strawhat had the same treatment. The only difference is that they had arc that were needed to introduce them and create their characterization. There is no need for that anymore. Strawhats are still relevant, they are just not the focal point, just as they were not the focal point during all those arc for other characters.