social – How can I, as a GM, make players with different playstyles be more moderate?

I’m a GM for this group of players, and we’re playing an adventure where they are supposed to be city guards. Now, one of the players is taking it to an extreme level of seriousness which, while in theme with the adventure, is in contrast with the others, who are a bit more light-hearted (some much more light-hearted).

A couple examples to understand the opposites: one of the most light-hearted players is playing a character that, while being a guard, doesn’t really have any business being a guard. No attitude, no mindset, no competence, and the serious player kinda has an issue with that (I suspect both in and out-of-game, but IDK).

On the other hand the serious player, while trying to get an idea about how to infiltrate a possible crime scene, thinks of any possibility (even becoming stuck on speculations or random details) and can take an entire session just thinking about how to do a thing (possibly without even doing it because of the repercussions that may happen)

Now we’re having an asynchronous conversation to try and see each other’s point, and while I’m trying to suggest each player to act in moderation, I don’t know if that will work. In that case, what could I do about it?