Wow, mad respect for this male lead. To those that feel that he changed way too fast, he was reading through the entire journal. He might have read it all in a single night, or he's been reading it for a long period of time.
But as he was reading it, he was questioning why the empress kept doing all the things that she did.
And the answer was that she did it to become the "perfect" empress.
As he was doing it and realizing everything that she did for him, he was beginning to realize that he deprived her of her own free will. He realized that because of him, she had turned herself into a mere puppet that was played with until she finally broke. And from the realization, he was hit with regret that's basically exploded from years of their marriage.
The speech he made to the sorceress was wonderful. I love how he just wants to make her happy and finally live a life of joy. He doesn't care if he dies, he doesn't care if the world is destroyed, he doesn't even care if she doesn't love him. All he wants is just for the girl that gave up everything, practically her own soul, for him to get everything that she deserves.
This is an ML that I can really get behind. The best part is that he's the isekai'd protag.