I would like to respond to a couple of points you made then state my own reasons for thinking this arc will be a long long one.
1) your first reason being that they've underestimated our guys, that's not 100% right. I mean yes, the locals don't know the strawhats but Hawkins, Drake and Apoo are all there and under Kaido's rule (I know we only saw Hawkins but I remember Sanji and Law's party mentioning Drake so he's most likely already there as well and for Apoo it's a 50%). These guys can already hold off Law and Sanji and maybe even another one of the strawhats... My point is: the battle against Kaido's army in wano (even without counting Kaido himself ) is going to be a hard one
Let's also not forget that Kaido is there for sure and also his first in command (the king guy that took down big mom's ship, that guy is probably on Katakuri's level so he is very bad news)
4) Responding to your 4th point, Kaido shouldn't be injured since he didn't fight big mom's crew. They were trying to get in but that king guy blew them away and no direct contact was made. So Kaido should be in his best form.
The only point I do agree with is that big mom will probably take part in the fight against Kaido because Oda-san wouldn't have made her appear there without a purpose.
For me, I think this arc is going to be long for these reasons:
1- Oda-san hyped up the wano arc for over 2 years and he kept fans on their toes so if he makes it a short arc that would disappoint most of his fans
2- Oda-san seems to follow a pattern where he makes certain arcs revolve around a certain strawhat character to emphasize their growth as a character (the arlong arc for Nami, the water 7 + Enies lobby for Robin, the big mom arc for Sanji...) and for the longest time, this was Zoro's awaited arc to shine.
Back in thriller bark, Zoro got the cursed sword and fought the samurai guy and Oda-san teased this arc
3- They have to succeed against Kaido in this arc (it doesn't have to be a direct battle but Kaido's claim on wano needs to disappear) because how else would the mink tribe and momonosulke, kiemon and the others fit into the rest of the story? The strawhats can't just sail away and run to the ocean with these guys because their whole purpose from the beginning was to raise a revolution and take down Kaido
Hmmm, true. I guess Oda-sensei wouldn’t really end this quickly because he is having so much fun. I just don’t think that they need to stay here for too long. If push comes to shove, this battle with Kaido will be fought mostly by the Big Mom pirates. The Strawhats would deal with Orochi and his minions. Of course, a tag team may occur between Big Mom and Luffy against Kaido. The Reverie arc just looks to be more interesting and more beneficial to the plot as of this point. The prophecy with the strawhat in Reverie still wasn’t explained and maybe a major war is brewing up there.
Since the battle between the Big Mom pirates and the Beasts Pirates was cut short and immediately showed an amnesiac Big Mom, I assume there was still a big fight that happened. Big Mom’s kids are for sure not wusses who just take a beating after their captain was thrown offboard. They must have put on a fight. If Katakuri was there, something might’ve happened that we didn’t see.
I also want to know Zoro’s connection to this place but I seriously doubt he came from here. How could a child possibly get out from such an isolated place? I do hope there is something story between the cursed sword he inherited though. That would be nice.
I don't believe that Zoro came from Wano not even for one bit but since he wants to be the number 1 swordsman and since he has been looking forward to fighting strong samurai since thriller bark, it's a significant arc for him (not in an "origin story" kinda way which was the case for Robin's arc, Nami's and Sanji's but in a "this is a place where he can have his fill fighting swordsmen").
Yeah, the big mom pirates will definitely play a big role in this but I have mixed feelings about that. I completely love Katakuri as a character and I want to see him again but I also don't because, after that epic scene where he asked Luffy if he'll be back, if him and Luffy meet again in a matter of days, it takes away the epicness of the whole thing... Then again, he might not even have come with the big mom pirates. It's a 50% chance ( she might have left him to protect her kingdom but going after Kaido without her strongest son is foolish).
Anyway, I guess we'll all have to wait and see what Oda-san has in store for us. Things are heating up
There are things that make me think that after Luffy and the crew starts there rampage, this arc will end immediately.
1.) Wano is a very isolated place. There could not have been any news from the outside world about the Strawhats. So they wouldn't know how to deal with them, much less know of how powerful these guys are. They're underestimating them so much that it's ridiculous.
2.) Orochi is afraid of the original warriors who served the previous shogun and yet these warriors were still overpowered by Luffy and the crew. What are they going to do with the massive power that is the Strawhat crew?
3.) The unrest in the country is great, it would be a no-brainer to think that the citizens would also join in the uprising once the plan starts.
4.) The only ones who could stop them on their plans would be Kaidou but I reckon he would be too busy or probably injured from the fight with the Big Mom pirates. Also, if they get Big Mom on the Strawhat team, it would be hell on Earth and the fight would definitely favor them.