I'm sure what I'm going to say isn't anything new by now. I thought this was a fine episode, on its own. Right off, Frakes brought an energy to it, and the pacing was pretty good throughout. But as the penultimate episode of the season, but especially, of the entire series, I found it wanting. It didn't feel like it raised the stakes enough. I wanted to feel like the crew were really in danger in this episode, like I have for several TNG, DS9, and even ENT episodes. A sense, of "oh s**t, how are they going to get out of that one?" For a quick moment though, I was wondering if they were hinting at offing Saru, and then on the Breen ship, I wondered if they would kill off Book, Rhys, or Adira. Saru is too popular to ax. I couldn't see them killing off Book because he hasn't resolved things with Moll yet, and I couldn't see them killing off Adira because I think they want to keep the family dynamic with them, Culber, and Stamets. That left Rhys to make the heroic sacrifice, but they kept him around too, which I was fine with.
I don't buy Moll as the de facto primarch. I don't think the series spent enough time building her and L'ak up to make me care about either. Maybe I could buy Moll more if this were the Emerald Chain instead of the Breen. As much as I was initially tickled that Discovery took on the Breen, they didn't need to. They could've built up the Emerald Chain more, created another enemy, though I would've been fine with them going with an even lesser-known species from Trek lore that would've given them even more freedom to play around with than even the Breen did. (I do like how DISCO has handled the Breen more than SNW have the Gorn though).
I thought the attempts to flesh out the Breen in this episode often went for comedy and also humanized them too much for my tastes. I still wish they had kept Ruhn around, or even just had his competitor already take over the Breen.
I thought Burnham having a heart-to-heart with Book on the Breen ship made no sense, but at the same time, it's such a Discovery thing to do that it didn't bother me all that much.
I liked the Rayner/Tilly dynamic in this episode, and I would be fine with seeing a spin off with Rayner as captain and Tilly as first officer instead of Starfleet Academy. I also liked the bit of banter we got on the bridge, though I still wish Owosekun and Detmer were there.
I don't know how long the series finale is, but it seems like they have a lot to get through, and I don't know if they will have the time to address all of that, especially in a satisfactory way.
I think this season started strong, but lost its way with the fifth episode, "Mirrors". I still find it more consistently entertaining than the fourth season, and I can't see it screwing up its finale as badly as the third season. I've been looking at Discovery's first season again, and I still enjoy it the most of any of the seasons. I wish they had kept the two mini arcs a season (that tied into an overarching arc or theme). I don't think they ever learned how to deliver on one massive story told over ten plus episodes. Season 5 is better than Seasons 3 and 4 in that regard, but it would've interesting to see what a Season 6 would've been like if they kept the big arc.