Frederick, I went over how Lahabrea learned from his mistakes in a thread about 3 months ago when you said exactly that same thing.
It's actually Igeyorhm that makes the mistakes in HW - Lahabrea is very precise and much more controlled in his behavior in 3.0 than he is in 2.0. The one time before the ARF that he actively interferes with you is the Vault - and by all rights, he would have succeeded had the WoL not had plot armor. Play through 2.0 and 3.0 again and you'll see how differently he acts regarding you.
Also keep in mind that 2.X is basically all about Lahabrea succeeding. Those Extreme Primals? He gets exactly what he wants from you with them. Primal Pet Project (the one two triggered the idea for which is very likely the WoL)? Completely successful - on two occasions!
Quoting myself from the last time you said this.
You're conflating underestimating a foe with not learning. It's not on him that our Blessing became a Super Blessing in 3.0 (possibly from Minfilia's sacrifice) paired with us becoming stronger as well. If anything, he didn't underestimate us in the ARF, hence why he has Igeyorhm with him in order to counter our strength, since it was clear that one Ascian is not enough to take us down. He was definitely more prepared that time. If anything, it was Igeyorhm who underestimated us. Twice. Once after Bismarck, thinking we wouldn't get the Blessing back and that we weren't a threat. The second time was in the ARF when she popped out her dark crystal early.
If it had been Lahabrea with Thordan after Bismarck, we probably wouldn't have survived the encounter. He wanted us dead; Igeyorhm seemed to just want to do her duty (also shown in their pre-battle dialogue in the ARF to further the evidence for this).
He did learn, precisely for the reasons I stated. He started taking advantage of the Echo in mortals (Ysayle/Shiva), when earlier he was basically just tossing its mortal usage away as weak. He found mortal use of the Echo useless until we beat him out of Thancred with it.
Lahabrea underestimating us would have been him coming alone to the ARF, thinking he could defeat us that way. Not learning would have been him continuing to view mortal potential with Echo as worthless. He does neither of these things.
So I'm not sure how you can say all he does is get a bigger stick. He changes his strategy entirely.