So...I finally gave this a chance figuring maybe the first movie was an anomalie. And thankfully...I was right.
Spoiler for Why the movie worked:
So while there is a bit of the usual banter and a reasonable amount of Mai, the movie really is about Kaede moving forward and the brother-sister relationship between Sakuta and her. Kaede is able to gradually handle everyday life in small steps. She wants to have a 'normal' life but people clearly scare her. Sakuta is incredibly supportive but silently, via Kaede's teacher Tomobe-san, it's clear that Kaede wanting to get into Minegahara will be very hard. With support from Sakuta, Mai and Nodoka, Kaede makes some big strides and the seemingly impossible looks plausible. That said, Sakuta does investigate alternatives and to his surprise, found that Uzuki ended up finding her way through a correspondence school. It's clear Kaede is hiding something but is always stopping partway when trying to say it. However, there is one telling scene with them and Mai. Mai is helping Kaede study and Sakuta is serving them dinner, but for a brief moment, he sees the Kaede that emerged for two years. Kaede asks why Sakuta keeps staring. Mai helps brush it off but it's clear something is amiss on both sides...
There is a foreboding scene where Kaede gets the courage to submit admission papers for the entrance exam. As she enters, Kaede shows desperation in trying to avoid being seen. Kaede does turn in the papers with Sakuta making sure she did get there. But as she leaves, Sakuta can tell Kaede isn't alright. Kaede can't hide the bruises/bleeding emerging from under her gloves from anxiety and how hard she's worked. Sakuta tries to reassure her but...you just can't shake the feeling...
Exam day arrives and Sakuta even makes Kaede breakfast and lunch. Kaede goes to the exam. Sakuta gets a call in the afternoon. Kaede went to the nurse's office during lunch break and won't go back to exams. Sakuta rushes to the school and finds Kaede under bed covers. Kaede did well in the first three exams but when another girl saw her during lunch - no words or faces - just saw, it's enough to send Kaede into a panic. Kaede hates herself for not being strong enough to finish the exams, saying that everyone helped the old Kaede and that the old Kaede was stronger. Then comes the critical moment - Kaede accuses Sakuta of preferring the old Kaede (aka post-hospital Kaede). It almost makes Sakuta lose it, including to the teacher who speaks to him right after. Sakuta collects Kaede's belongings from the exam room and finds old Kaede's diary. Wondering why the real Kaede would have that, Sakuta finds on the last page old Kaede's unfulfilled wish of going to the same high school as Sakuta. It all becomes clear.
Days after, Kaede wants to apologise to Sakuta but is still hiding something. Sakuta asks Kaede to come out with him for the day. They end up at a concert Nodoka and Uzuki perform at. Sakuta mentions to Kaede that Uzuki went to a correspondence school to find her way, which surprises Kaede and gets her admiration. Uzuki is going to speak to Kaede after the concert. While waiting, Kaede brings up the fact she thinks Sakuta prefers the old Kaede. Sakuta admits he was sad that Kaede is gone but was more happy that Kaede came back. It's the moment things are finally fixed between the two.
Uzuki details her past issues to Kaede and Uzuki's mother admits her own side of it in wondeing whether it was her fault. The core message from this scene is that Uzuki and her mother had made up their mind that doing what everyone else deemed right wasn't the way to go and that while Uzuki says she is in a better place now, the old her was still part of becoming who she is now. Kaede is heartened by this and while she did end up passing the entrance exam for Minegahara, she ends up going for the correspondence school and ends up re-connecting with Komi, one of her old friends. Kaede now feels she can finally move forward. This take on the school system and not conforming to what the majority/society deems the right way - it's very daring given the societal norms in Japan. This film made a good case for individual needs not always being met by the norms and to find the right way for the individual.
Final scene after the credits - Sakuta laments this being the last day he'll see Mai at school. Mai asks him to meet her at the beach. And after he waits, all the dreams he's had during the movie become real...young Mai with a knapsack shows up..the obvious lead-in to the Knapsack Kid film.
So...I know what I said previously...but in all honesty, Venturing Sister is like Dreaming Girl never happened aside from one early throwaway line. I still don't forgive Sakuta for what happened in that first film - but yikes, he really delivered in Venturing Sister. What happens between him and Kaede is close to the best 'arc' of the title (Mai arc is always best arc). And now I have to see the Knapsack Kid film at some point.