Voltron evolved into Magneton shortly before the Plumeria battle. After fighting Olivia, I brought it to Sun via Pokebank and had it evolve in Vast Poni Canyon there because I didn't want to wait on getting that Magenzone. Makoto evolved into Primarina right after the first battle with Hau on Ula Ula Island.
As for the fights and story progress:
1st Plumeria battle: The same as in Sun/Moon. She still uses a Golbat and Salandit against you. I had an easier time with this battle, but that's because I figured out the best strategy against her Golbat. Turns out Magneton really shuts it down. Only Confuse Ray and Bite really affect, but the latter won't do much damage to it. After Voltron got confused, I used Thunder Wave to ensure it would outspeed Golbat. From there, I managed to land some Thundershocks to knock it out. Salandit went down very easily to my Makoto with just a single Bubblebeam.
Grand Trial vs Olivia: Remember that really annoying Nosepass she had on her team? It's gone now, and so is her Boldore. Now, the two Pokemon are replaced by an Anorith and a Lileep respectively. Even without her Nosepass, she was still not easy to take out. I led with Siegfried against her Anorith, and while Siegfried delivered a strong Brick Break onto it, Anorith managed to 2-shot him thanks to a critical hit from a Bug Bite. Voltron came in to tank Anorith while I healed up Siegfried, and then it finished Anorith off with two Magnet Bombs. Her Lileep was next, and Siegfried took it out in two hits. It does know Ingrain and Mega Drain though, so be sure you hit it with some very strong hits or else it will take a while to take out. Last came her Midnight Lycanroc, and that Pokemon got lucky with flinch hax on Makoto thanks to Bite. It did use its Z-move once, but Makoto lived and managed to respond back with Hydro Vortex to finish it off.
Vs Hau on Ula Ula Island: Unlike Sun/Moon, he now has five Pokemon on his team at this point in the game, and for this battle, Tauros is a new addition to his team. Like the past rival battles, he leads with his starter, so be sure you start the battle with the starter you have. Noibat and the Eeveelution he uses depending on your starter were also not that hard to defeat. Now, remember that nightmarish Alola Raichu on his team that could sweep your entire team clean if you weren't careful? Yep, it's back! I tried to knock it out with Sirius, but Sirius went down very quickly. Luckily, Siegfried took care of Alola Raichu rather quickly with Night Slash (as well as good Special Defense to tank most of Alola Raichu's attacks). Tauros was arguably the most difficult member of his team to take out it. It deals some pretty strong damage with Horn Attack, which took out Siegfried rather quickly with it. When I brought back Sirius, I was shocked to learn that Tauros knew Pursuit as well. Yep, it even has coverage against Ghost-types, so trying to nullify Horn Attack with a Ghost-type might not be the best strategy for this battle. Voltron, now a Magenzone at that point, had to finish the Tauros by tanking it.
Story updates: There's a pretty major story change during your first visit to Aether Paradise. After beating Nihilego, the Ultra Recon Squad arrives and tells you and Hau about Ultra Beasts and Ultra Wormholes. Hau is visibly weirded out by the two, and even takes on a thinking-like pose with a serious face looking at them. Lusamine formally introduces the two as part of the Ultra Recon Squad, and she reveals that she has been working with the Ultra Recon Squad to fight Necrozma in order to protect the Alolan Pokemon, fitting for her "motherly" dynamic she has with them. Also, they completely removed the scene where Lusamine does that creepy smile that says "I'm totally not the main villain in this story, am I right?" while talking about how she needs to "love" and "save" Nihilego. By removing that creepy smile at that point in the game, USUM does a decent job making that whole scene much more subtle regarding her reveal later on.
Also, when you and Hau talk to Kukui in Malie Garden, Hau is the first person in the game to refer to the Ultra Recon Squad as "aliens from another world." I'm glad I'm not the only one that thinks the URS are technically the first extraterrestrial trainers in the series.
As for the Kantonian gym, like I mentioned, it is located in Malie City, fitting considering how the city is modeled after Japan. Despite being modeled after the Vermillion Gym, it does not utilize the trash can puzzle at all. The trash cans that are there are purely aesthetic. To fight the "Gym Leader," who is just an ordinary Black Belt, all you need to do is fight the four Beauty trainers first and that's it. Surprisingly, Ryuki didn't show up in the gym despite appearing in the trailers. I guess he's a post-game deal?
I am now on Route 10 and will be taking care of the Electric Trial later today.