Admiration is the furthest from understanding

Sorry it took me so long to answer this. I had to think about how to word it.

I absolutely think Aizen is as brilliant as advertised. However, as soon as his decades of elaborate plans start actually working, he fell victim to the enemy of all those who would challenge gods: hubris.

select excerpts from Wikipedia:

Hubris… describes a personality quality of extreme or foolish pride or dangerous overconfidence, often in combination with (or synonymous with) arrogance. In its ancient Greek context, it typically describes behavior that defies the norms of behavior or challenges the gods, and which in turn brings about the downfall, or nemesis, of the perpetrator of hubris.

…Hubris often indicates a loss of contact with reality and an overestimation of one’s own competence, accomplishments or capabilities.

…A common way that hubris was committed was when a mortal claimed to be better than a god in a particular skill or attribute. Claims like these were rarely left unpunished, and so Arachne, a talented young weaver, was transformed into a spider when she said that her skills exceeded those of the goddess Athena. Additional examples include Icarus, Phaethon, Salmoneus, Niobe, Cassiopeia, and Tereus.

…The proverb “pride goeth before destruction, a haughty spirit before a fall” (Proverbs 16:18) is thought to sum up the modern use of hubris. Hubris is also referred to as “pride that blinds” because it often causes a committer of hubris to act in foolish ways that belie common sense. In other words, the modern definition may be thought of as, “that pride that goes just before the fall.”  

In this way, Aizen is a modern example of the age-old cautionary tale about the downfall of those who think themselves superior to gods.

In his secret planning stages, he was meticulously careful with every detail. He did well at the “haha I’m doing this right under their noses” part without blowing it; I think it was due to lack of reaction/feedback/immediate gratification by his enemies. He felt powerful by virtue of moving in the shadows unseen and playing with powers others had no knowledge of. I think things changed for him when he “went public” in the Soul Society arc. When he started seeing open reactions and fear and an entire spirit military scrambling to figure out how badly he’d torn them apart from within and how to fix it or work through it. That had to be a great high. It was a new kind of power… and it was like a slow-acting poison.

I do think he did maintain some decent strategy for the battle, but I think he also allowed too much toying around and let himself have too much fun traumatizing shinigami when he could have gone for the goal faster. It’s been awhile since I read it, but I remember noting that Aizen had a lack of urgency during the whole ordeal. I think it was because he got too convinced that his plans were so awesome that struggles against them were pointless, so he found it entertaining to observe the battle like watching a hamster running in a wheel. “Haha, look, it thinks it can actually get somewhere.”

Even though Kubo had designed him and wanted to feature him, he hadn’t figured out where he would fit - seemingly until crunch time.