I think it’s looking pretty good, I’d stay away from doing such a drastic pose though when you’re first sculpting, I’d nail all proportions and such, and then pose at the end and do any fixes you need to then. That will give you the ability to do a few different presentation poses if you want and it will be easier to do that with a more neutral starting position.
I tend to disagree with Sawarane though for the most part (keep in mind this is all opinion). First of all the anime isn’t the original source material the manga is, so if you were really worried about that, that’s where I’d go. And honestly that’s not just a 3D print that “some other sculptor did” it’s a professionally sculpted piece that’s licensed and approved by their production company. I do still think that referencing someone else’s sculpt on something that is 2D originally is a good way to get an idea of how you want to interpret the forms, there is no right or wrong, if you don’t like a certain way they interpreted a form, don’t reference that part.
I don’t actually like that sculpt you’re looking at particularly, it’s okay but it’s not the best. Check out the Bandai Figurarts Zero figures (My favorite personally, and I think they interpret the characters the best) and the Megahouse Portrait of Pirates sculpts are also pretty nice.
But, that’s just my two cents, keep workin’ on him! Oh and I definitely prefer post-time skip Luffy, that big scar is a cool character addition! ha.