I remember watching Descent when it first aired and I will say that I agree with parts of your comments and disagreed with other parts.

The cold open of Descent Part 1 (D1) was great - very intense and thrilling. However, I felt that Descent started to get undermined as a great episode even within D1.

BOBW 1 focused almost entirely on the Borg threat, the crew preparation, the attempts to stop the Borg as they rolled towards Earth in a countdown-clock threat situation. I will admit there was a minor character element of Riker vs. Shelby and Riker's refusal to accept a command, but that was somewhat organic and related to the Borg threat, because his command would have put him in charge of a ship that was destroyed at Wolf 369, and Shelby is a borg expert who is ostensibly onboard for that reason - it just happens that the Admiral wants Picard to convince Riker to accept a command and take Shelby as the new XO for the Enterprise. It also doesn't take up a whole lot of the episode's real estate. It's more underlying tension during plot-related scenes.

The focus on Data's emotions comes off as less organic and integrated into the plot - Data's emotion chip is, by definition, artificial, and so his focus on suddenly feeling anger feels like a plot element invented for the sake of plot as compared to Riker and Shelby's credible feelings and reactions to a believable promotion opportunity.

it feels like D1 quickly becomes less about the borg, the imminent threat, and the tension of the Federation itself being destroyed, and more about Data feeling emotions.

The borg themselves showing sympathy for a fallen borg comrade is interesting, but also undermines the borg as an unstoppable emotionless assimilation squad which is what made BOBW so exciting.

Then we get a scene where Data discusses emotions with Troi and LaForge where in season 3 we might get some very complex and introspective dialogue, but in season 6, we get a very sesame-street feeling exchange right down to a "for example" - sometimes that just doesn't feel like natural spoken dialogue to me:

DATA: Anger is a negative emotion. I wanted to concentrate on something more positive.

TROI: Data, feelings aren't positive and negative, they simply exist. It's what we do with those feelings that becomes good or bad. For example, feeling angry about an injustice could lead someone to take a positive action to correct it.

Then when pressed, Data realizes that not only did he feel anger a borg, he also felt "pleasure" at killing one. Again, this could have played into a much darker tone for the episode like BOBW or First Contact, but it ultimately doesn't pay off in terms of the darkness. Perhaps the crew should have had a discussion about whether Data might be dangerous if he is feeling pleasure at killing or something like that. Maybe there should be some foreboding signs that Data might be a bit unstable.

This is followed by Data testing out his emotions by emotionlessly throwing a borg drone against the wall repeatedly in the holodeck doesn't take itself seriously enough and comes off as a bit of a farce.

We also get a scene where we have a red alert based on a distress call that turns out to be a colony spooked by a Ferengi ship - "third time today" - it again undercuts the tension in the episode and makes it feel more like a mid-season procedural and less like an event-level cliffhanger.

When we get Data confronting Crosis in the brig, I can't put my finger on it, but the exchange just doesn't have the "sinister" factor that Data's discussions with the Queen have in First Contact - there is no atmosphere in the room in D1 (also, if I'm not mistaken, there's still security officers literally in the room listening to the exchange who apparently don't intervene or report the conversation to anyone... it again undercuts the realism of the exchange).

Finally, we don't get really great faceoffs against the borg where we see that our crew is in serious peril - it is more of a cat and mouse pursuit with the borg running into their subspace conduits - this again makes them feel like of a threatening (BOBW) and more of a frustrating.

I did like the idea that the ramifications of Hugh in "I Borg" could actually be addressed, and I'm sure there was a way to make those ramifications result in an even scarier Borg such that 30 years later we might still be having conversations (akin to Tuvix or Pale Moonlight) about whether Picard did the right thing in that episode or the wrong thing. But at least in D1, we don't see the Borg necessarily become an even bigger threat than they were before. So I don't think it pays off even in D1. They try to give this payoff with Admiral Necheyev, but I don't think it really lands like it would if there was a more dire threat.

The payoff that Lore has seemingly teamed up with the Borg is admittedly a great cliffhanger moment... but the rest of D1 just doesn't quite live up to BOBW 1.

Of course, you're right that D2 certainly took what was still a reasonably entertaining D1 (not epic, but entertaining) and didn't pay it off terribly well, which happens with 80% of two-parters - but yeah, the Picard/La Forge/Troi in prison scenes are not overly effective to me. It pales in comparison to Garak, Worf and Bashir in Dominion prison in DS9 which was executed far better. The whole Beverly in command sequence feels like a rehash of Laforge in command in Arsenal of Freedom (right down to the officer in charge encouraging an unassured crew member, and dealing with another crew member who is overly arrogant and critical), which was fine for season 1, but it too simplistic and rudimentary for season 7.

And while the groundwork was laid in D1, D2 really confirmed that this new collective was a shadow of the old borg and barely keeping themselves together. Hardly a megathreat like the BOBW borg.