Having digested a little more, it seems like the main purpose was to get a two-hander (moreso with David and Catherine than the Doctor and Donna), but that kind of became the only purpose of the episode. The situation essentially resolved itself, the Doctor and Donna were totally unnecessary and everything would've turned out fine if the TARDIS hadn't ended up there (indeed, possibly slightly better, with the Doctor being concerned that he may have accidentally conjured up interdimensional space vampires while bluffing with the salt). The TARDIS popped back on its own to save them, and the TARDIS was the one who figured out the Doctor picked the wrong Donna to save (which seemed oddly sanguine about in the end, it feels like that should've been the kind of thing she'd give him shit for). They figured out what the story of ship's captain was, which I guess is a valid mystery-structure for conveying a narrative, like in a short story, but we still never found out what the ship was, if there had been more than just the captain on it, or anything else beyond how someone else had already encountered and resolved the threat before the Doctor and Donna ever showed up. Even the Doctor's resolution, such as it was, was just to speed up what was already happening. The Doctor and Donna didn't solve any problems or come to any deductions, everything was done for them. It was just way too easy.
It was stylish, and what was there was well-executed, but for a decennial anniversary, and coming off of a period where the Doctor was written as generally being uncharacteristically passive and powerless, I don't think it lives up to what it should've been. Even for implications outside the episode itself as a standalone, as has been mentioned, the real Donna knows (or is willing to say) nothing about the Flux or the Timeless Child, and the Doctor seems to be back to his worst impulses with lying to his companions and suppressing his emotions and concerns, despite the big talk about the Fourteenth Doctor being a different, more mature version of the character than the Tenth.
I could probably say more nice things about the episode, but it would all be in the context of disliking Chibnall's writing, and that would turn into preferring Moffat's, and that'd be unfair to everyone. So, just stick with the broad notes: Run the "Indiana Jones" test (apocryphal, but useful), and think about how your plot would be different if your protagonists hadn't shown up at all.