Finally got a chance to watch the episode and wow, what an opening. Lots of moving parts that I'm hoping will gel together as the season progresses. For the most part, the main focus is fairly clear with the Flux as universal destructive force that appears to be controlled by Swarm, apparently an ancient enemy of The Doctor's, or more likely, the Timeless Child, considering The Doctor doesn't recognize him.
The mad dash introduction of Dan via Karvanistra who was similarly introduced quickly as a villain but is now a reluctant ally fits the swift pace of the rest episode. We're also rapidly introduced to some 19th century tunnellers, classic Sontarans tracking the Flux, the outpost watcher Vinder who we already knew was going to be important, Swarm's sister who was in hiding not unlike The Fugitive Doctor, and Claire, a mysterious woman who The Doctor and Yaz will meet in their future who is being chased after a Weeping Angel. Oh, and Dan's not-a-girlfriend friend, Diane, who gets kidnapped by Azure in perhaps a coincidence too many.
I really enjoyed the rollercoaster ride and I might rewatch the episode again ahead of next week's episode to see how well everything fits together. Lots of mysteries run around, not just the Flux and whatever Swarm's motives might be, but also The Division that The Doctor is so concerned about, which may or may not also tie in with the Timeless Child.
Perhaps this is just me, but when I first saw Swarm and his changing crystal form, I immediately thought of Eldrad (especially since there were two different actors playing him). Considering The Doctor doesn't seem to recognize him, I'm guessing it's nothing to do with Eldrad but the similarities still stood out.