I never understood why she is due to the reasons you mentioned, her skill(s) overlapping with Robin and Sanji, and her showing no combat prowess whatsoever.
I never saw her having potential as soon as Jimbei showed up in the arc (not that I didn't expect him to appear somehow anyway), but seeing her Devil Fruit in action completely deconfirmed her for me. However, she did stand out due to being foreshadowed in Fishman Island and Luffy saying that Sanji's wife can join the crew if he gets married in Zou, which isn't too different from how Luffy kept saying he wanted a musician for years on end until Brook appeared.
Usopp did draw the "masks" for the four SHs with switched souls, and his own face for Kanjuuro to use as a reference against Sugar.
I completely forgot about those instances. That's cool!
I can understand why some people are going on and on about Oda's patterns, but there's no reason why he can't change things up much the same way he did with Robin. He also mentioned how new characters would join the SHs successively and if this is Jinbei's time, the last crew mate will join after the next arc. I and others may be wrong, but given what I've seen and all of the variables at play including where the SHs will be after Wano, I can't see any other characters unseating Carrot as the best potential last SH at this point. Pedro being alive will make me second guess myself, but for now, I'll stick with Carrot as my guess until I get more info.
Aside from Robby concisely replying to your points, the problem is not Oda wanting to deviate from his patterns. The problem is Oda deviating from his patterns in ways that present redundant traits already shared by the crew and downgrades of special moments we come to expect for each Straw Hat (like how short and uneventful her flashback was, or her relationship with Pedro being near non-existent and unmemorable, or Pedro's dream being so vague and thus unimpressive). He subverted patterns for Robin, but she still had unique qualities (differently drawn beautiful female face from Nami for the first time, unique Devil Fruit, standout archaeologist knowledge, was a shady foe who seemed dangerous yet mysteriously saved Luffy's life, and had some sort of secret agenda that would later be explored).
For Carrot the only weird think that needs resolution is her need for adventure. She became to much of a recognizable face to just be a mink in the tribe and for her finale to just be there. And while I didn't see a satisfying way for it before(I doubt she would become a crew member) I think the flashback does help with that. I picture Pedro once again sailing a pirate for in order to help he strawhats accomplish their mission and Carrot will be part of it. This woud basically be the same ending as the one Margaret got. It doesn't do much for Pedro who would't accomplish much with this new crew but it is perfect or Carrot.
I expect her to set sail with the Nox Pirates or some new crew as well to conclude her character arc. The big question though isn't if Pedro is alive (he is) but what role Pekoms might play.