Something I really enjoyed was how they went back and fleshed out the minor characters from last season. I went in assuming Martha was going to be all but phased out once Jonas went time traveling, but she's a focal point of the whole thing. Old Jonas became more of a character. Elisabeth got a meaningful, albeit not huge, story segment. They brought back old Mikkel. Katharina might not have had a purpose, but she had purpose throughout, and it was compelling enough to keep her in play for next season without having it feel like she was spinning her wheels. Even that Woller dude got some elbow room. It really solidifies the whole card tower of events when characters you thought were minor get fleshed out in important ways. Contrast this to something like Stranger Things season 2 where they bloated the cast and left everyone feeling undercooked.
About the only thing I didn't like was Ulrich's failure to gain any perspective over his three decades of imprisonment, and even that's only half "meh" and half me just being disappointed.
I'm still not sold on Jonas being Adam. Old Jonas must've known that he was creating Sic Mundus when he saved Magnus, Franziska, and Bartosz. And yeah, that makes it make sense that he would be so much older and decrepit compared to them as Adam, and he does refer to Bartosz in the third-person, but we never see Bartosz in the future. We don't see Jonas or Claudia experience any detrimental effects comparable to what happens to Adam as a result of traveling - and Claudia traveled a hell of a lot, a year passed between her and Jonas leaving Old Mikkel and Jonas returning to middle-aged Claudia. What we do see is that Bartosz's family has a history of lethal, aggressive cancer. There's some funky wiggle room shenanigans in there between Jonas leaving her in the bunker when he knew she was going to die and him saving the trio with the machine. So, I'm still leaning towards that malevolent little shit Draco Malfoy wannabe as Adam.
The ending wasn't bad, but I'm a little flat on it. For a show that so arduously, meticulously works to weave everything together in a structurally sound way, I was hoping for another, grander twist in the weave. Instead, we get a parallel/mirror reality tearing through it like a bullet. Which is fine, it's always been alluded to with the opening, and it does work. You can't break the loop as part of the loop, so alternate realities/timelines are the only way you can actually justify a time-altering Terminator-style storyline. I'd just rather have seen them add another dimension to the master plan of the show, rather than bring in an element that gets to wipe its ass with the master plan. It wasn't cheap, it's just a little... mundane. Straightforward.
Maybe a little cheap, considering how intricately woven the rest of the story is.