Very nice theory. I like the overall idea, you structured it very well, and your text is easy to read.
About the general idea here: I agree with most things said, tho you lost me a bit at the broomstick part, partially because I personally simply dont like the idea of Nami floating around on a Klima Tact powered "broomstick", and partially because I dont see it fitting. For one reason because the Klima Tact is more of a weapon than a universal tool, and for another reason because the "witch" theme of Nami is completely related to her weather-manipulating abilities, like all your arguments pointed out. A broomstick however is not related to weather at all, it's simply related to the traditional theme of a witch, as found in fables, fairy tales and such stories & fiction, belonging to other typical witch things like boiling cauldrons, black cats, long noses etc. So the whole broomstick idea stands out as vintage witch example, not fitting the remaining weather-manipulating and thus "magical" examples.
Tho it could still happen ofc, a theme doesn't have to be 100% consistent. A reason you didn't list for why it could happen especially with Nami, is that she already demonstrated her skill in handling the wind/jet dial (Nami's Waver). A Klima Tact broomstick with wind jets as "engine" would function with the exaxt same principle, so it's easy to imagine Nami being able to handle the coordination needed when flying a wind-boosted broomstick. ^^
Another thing I'd like to add to your theory is a pretty obvious one: The people on Weatheria not only have the abilities of a "Weather Wizard", but are actually dressed like standard wizards as well, giving concrete proof to the whole weather wizard idea. Maybe you found it too obvious to point out, or you implied it in your "Weather Wizard" sentence, but for the sake of completeness, I thought I should mention it.
All in all, I certainly agree with it, or rather: It's not difficult to agree with, since there is nothing far-fetched here, it actually feels more like an advanced and with well-researched info enriched summary, because Nami's witch theme certainly seems to be true. Basically what you say is that Nami is witch-themed and thus a Weather Witch already (or on her way to being one), and that she'll be the most skilled weather-manipulating human ("witch") in the series, which is definitely a safe bet, and you do a great job of pointing out her witch theme.
One thing I miss a lot in your theory though is what Nami's power-up is exactly, and where it will take Nami. What exactly is her future wizardry & witchcraft, her abilities? You talk about her next power-up, without concretely saying what that'll be. Just an improved Klima Tact? More knots? You get what I mean, I'd like to see where you take this theme with Nami concretely regarding her abilities. ^^ No offense here, but 99% of the fans would expect an improved Klima Tact as upgrade, and 60% would expect the knots as she used them on Weatheria, so when I read your title "Nami's Next Power-Up", I expected to see concrete suggestions of what her power-up will be. The only new suggestion here is the idea of Nami using her Klima Tact as a broomstick, which is not really a power-up in that sense. What you really do, for >80% of your text, is pointing out Nami's witch theme (which you do in great fashion), but you only talk about her actual power-up in 3 sentences, all 3 of them describing abilities fans expect from Nami anyway, which is why I would say "Nami The Weather Witch" is a more fitting title, but that's just my take on how the title gave me expectations different from the actual text.
About the last bit, your speculations part, I'm not sure I buy the Devon Witches inspiration yet, but for now it's a nice link. I'd add to that the fact that Cat Devon uses a spear as weapon, which in size would fit for a fight against Nami's Klima Tact. About her having killed Nami's parents: Would be awesome. I like these past connections a lot, the more drama, the better. ^^
PS: Tho I gotta say, as I said before in related discussions, that I find Nami's abilities to be somewhat boring. It's a problem with longrunning series if the protagonists always fight the same way. Luffy punches, Zoro cuts, Sanji kicks, Nami summons clouds... I'd prefer if Oda mixed it up as much as possible. More Conq. Haki abilities for Luffy, different cutting techniques (like Kinemon cut fire) or a sword with a DF for Zoro, longrange attacks / Rokushiki for Sanji, and a DF for Nami. We still have ~30% of OP to go, and just Klima Tact in battle is not necesarily something I look forward to, tho I do like the witch theme, which is why I'm curious as to where you think this will take Nami in terms of diversity and abilities.