The goofy personality must have been Sylvester's real character, but he can't act like that in an official capacity as the lord of the domain. He wouldn't be taken seriously, which would lead to assassination attempts and wars. When he's a ruler with absolute power, he also needs to use the absolute power for it to stay absolute. Dictatorships always suffer from corruption, but it also depends on the ruler's behavior how much corruption thrives. It's hard to imagine this would have been the first time the high priest was attempting to fill his own pockets by abusing his power and position. So, the goofy character disappearing at least sometimes was inevitable as soon as Sylvester was revealed to be the lord.

I'd say the harshness of the deal was to be expected. Normally Main would have either died because of devouring or become a magic generator for a noble, basically little more than a hidden slave locked away. Sylvester might have absolute power as the lord, but he can't go against all the nobles in the domain. He needs to follow the same general rules and laws and respect the traditions to a point. He needs his own faction of supporters. This was probably as much as he could stretch things for Main's sake. A whole lot of talent will be wasted in the city, but that's how deeply hierarchical systems work. Maybe Sylvester actually plans to change things relying on both Main's enormous magic capacity and the printing press. It would mean at least partially destroying his own position and reducing the importance of nobility, but if it succeeded, it would make the domain a lot stronger. We can only hope.