As a whole, I like the plot going through these two DLCs, but will echo the sentiment that the Area Zero segment feels very tacked on for being the thing the DLC is named after.
Gonna get the negative parts out first. For me the AZ issue is a combination of just kind of being tacked on after the Kieran League story, and being very barebones on a workable-enough skeleton premise. The inception is just kind of you being told after winning the League that Briar wants to go investigate and only just got clearance, and so she wants to take 4 of the students along. The thing that confuses me is her choices: Putting aside your main character status, they only just learn you're the Champion at the meeting, so unless it's because you're a Paldea Champion it feels very plot-pushy. Kieran/Drayton as the then-current/former Blueberry Champ tracks fine for me, but then Carmine is stated to not have been keeping up with League Matches while not having a secondary reason like Penny's alleged computer skill or Arven's knowledge of the Professor/Book. I would have assumed she'd invite members of the Elite Four, since Kieran's invitation indicates it's not a matter of role or authority needing them to stay.
Terapagos also just feels very flat. You walk through several rooms with a couple scripted fights against Stellar Mons, and then find it at the bottom. Then you wake it up and it just kind of starts running out of control and your characters are convinced you have to beat and recapture it immediately. The thing is unlike Eternatus, I don't get the sense leaving Stellar Turtle here is going to have consequences for the region, nor does it exhibit a behavior that says it's gonna try to kill you for leaving like the story Professor and their Raidon. It kind of wants to recapture the main story finale but without the set up or build up, when I honestly think the Kieran stuff and maybe Pecharunt would have sufficed. Not to mention hanging questions like why they point out the Terrium's big Crystal if it just... is there with no real relation to the Area Zero stuff like a Key to the lower area or something.
I think this could have been much improved by a couple rewrites: Kieran already wants to go to capture Terapagos in the current plot, just have him go off to do it himself after learning about it from Briar debriefing an expedition. Going in pursuit of him is a much more coherent reason to bring you and Carmine in particular, and it makes Kieran's turnaround feel more genuine when the action he goes too far with is entirely of his own making instead of being instructed by Briar, plus the whole "Hidden Treasure was the friends we made along the way" message instead of getting stronger like he assumed he had to.
On to the Positives, which much outweigh the negatives but by nature won't be as wordy. The difficulty was simultaneously harder than I was expecting but still totally manageable for my approach of "keep 1-2 old mons and otherwise rebuild my team" (my keepers being Skeledirge and Ogerpon, newbies were Excadrill, Hitmontop, Reuniclus and a rotating slot with Metagross by the end). The Doubles nature and opponents employing strategies that didn't depend on 2 AI decisions so much as just Team composition and mates (which I think hindered Colosseum and XD at times) made me have to think more on my feet, plus some team mixups minimizing type-based facerolls. Kieran even managed to force a rematch out of me even with item usage when I went into the first attempt with suboptimal leading. Honestly it might have even given my in-game team trouble despite their being in the 90's, if only because Doubles structure doesn't lend as well to outpowering a 20 Level gap alone.
The new characters I generally liked. Amarys was the least interesting of the new Elite Four to me just because her animations didn't feel stoic so much as stiff to me, and it's just not a character trope I tend to get into, on top of not finding it a good fit for Carmine's friend concerned for her brother as an emotional issue. Crispin's pretty one-note, but it's an energetic one-note and I like his cooking motif visually. Drayton I can see people liking, but I felt a little confused on if I'm supposed to think he's an asshole or not: one angle is he's concerned about his club and wants Kieran to lighten up as the game plays it, but his smug and somewhat lazy mannerisms also (unintentionally or otherwise) make it feel like he's using me to take his rival down a peg since he can't win against him himself. Lacey in turn is the Elite Four Member I enjoyed the most, having easily the most creative battle team (Excadrill being an excellent mon and choice for a "Tera into my Type" Ace alongside the Clay theming) and the most dialogue of the Main 4.
The Terrarium was a neat map, liked the variety of Pokemon to find and thought it benefitted a lot from more defined setpieces to figure out where to look for Pokemon like the Maze for Diglett or the Chargestone Cavern for Electrics. Also nice having stuff like Tera Shards lying around everywhere, and the Item Printer is a great QoL update for stuff like getting Special Pokeballs instead of resetting auctions like I had to to get a Dream Ball for Palkia.
In essence, I'd say the DLC is a 7.5-8/10 Pokemon experience, hampered mainly by seeing how easily it could have gotten up to an 8.5 and just didn't on those spots.
