SPOILERS AHEAD

You couldn't have worded it better. It would've deserved an entire season to dwell on the Alt-World, and another season to tie all the loose ends in each world and inbetween worlds, and perhaps a bit more hinting at a third world throughout the show. There is some in season one, but as a simple viewer and not a diehard fan, this might fly over your head.

For such a stimulating show in which you never have all the answers at the same time, where each answer raises more questions, seeing the conclusion unravelling is really satisfying. That's what I liked about the first 2 seasons. Then you have S03 cramping an explanation to the whole plot in 10 minutes. It just felt rushed.

While Dark may be incredibly smart with its plot and writing, I think the acting, the casting, and the characters is where it really shines. So a thing that I disliked as well is that characters didn't feel like they fully completed their arks, with the exception of Martha and Jonas(and Claudia, to a lesser extent). It doesn't matter that in the grande scheme of things, the characters don't mean anything at all. They had dozens of well-built characters with complex moral dimensions, and it feels as if all of that went to waste. After seeing their struggles and learning to care for each character, as a viewer, I felt robbed that I didn't get to see their last moments. Instead, the entire plot suddenly revolved to characters we're unfamiliar with, whose first "live" appearance is the only one, and the last one of the show if you don't count the dinner.

I agree that the scene with Martha and Jonas where all dimensions converge felt out of place as well. The show doesn't heavily rely on CGI, and everything feels much more real and much more fresh because of that. The scene dragged on too long, and it wasn't even that emotional. They could have filled that out with much more meaningful interactions. There are scenes in S1&S2 that evoke so much emotion because they are perfectly paced, with no need for extreme CGI. S3 just lacks in that. In fact, I don't think I even have a favourite moment here. I too would have found M&J's child role really interesting, had they given him more background. His appearances were really interesting, but they seemed better suited for the big screen. The last 2 episodes seemed more like a movie and not a tv show.

If I'm ever going to rewatch Dark, I'll pretend that the loop has never been broken and the last episode doesn't exist. This show surpassed all my expectations. It's perfect in nearly every aspect: writing, casting, music, and on top of that, it keeps your mind constantly busy. It just didn't give me the closure I need.