Just finished it, so just off the cuff thoughts here...also have not read the books. First off, while I enjoyed it, was easily the weakest season for me, which seems consistent with quite a few opinions I'd seen about the 4th book this season was based on. I can understand there needing to be a rest/reset to start setting the stage for the next major arcs, but the reduction in scope on both the alien and political fronts felt pretty jarring.
The whole Ilus situation felt like it should have been resolved in 3-4 episodes, as the main issues were the humans fighting and the alien tech...yet they kept piling on more issues that took until the very last minute to solve. Maybe it wouldn't have felt so stretched out if things had been more interesting on the character development side, but outside of Amos, nothing I recall really stood out for me, especially among the original cast. Without spoiling, will also say I didn't find most of the story with Avasarala that enjoyable either. I did like that her fiery personality got dialed up, but the governmental scenes this season paled in comparison to the debates and war room scenes previously.
Lastly, maybe my biggest plot nitpick was right off the bat, with how the whole shitstorm started on Ilus. Maybe it's explained better in the books, but after Earth and Mars both almost get wiped out by proto molecule, and then the entire system almost blown up, how in the hell did all the governments not have at least a full battle group of ships guarding the ring entry. Humanity was almost wiped out, and now there's over 1000 new systems, with who knows how many possible things that could result in a catastrophic event to be unleashed. I know the issue was a talking point for a few scenes, but it was mostly hand waved off. I get that raging tribalism and thirst for possible riches are major driving forces, but man, seems like there's a big shortage of leaders with common sense in the future.