@Scrabbleman Trope of isekai, MC protagonists are incredible idiots when the plot doesn't demand they be geniuses. Or rather, they're huge idiots when the plot demands it.
Despite that it's the most obvious and intelligent way of handling things, even from Rimuru's perspective and knowledge, Rimuru won't think of it - either until it's too late, or at all. There's literally no reason to keep it a secret at this point.
Not that it'll matter. As a developing kingdom, other kingdoms will seek to destroy it and take its resources before it gets as powerful as they are. If Tempest gets as big and strong as this other kingdom, then Farmus or whatever will have to start dealing with Tempest diplomatically. And let's not forget, diplomacy is just basically "Talking things out while reminding you my words are backed up with armies." It's the respect given out of fear of mutual destruction. Which, when we're dealing with these cartoonishly greedy and proud rulers like Farmus' king, would bring great shame when they have all these corrupt reasons to believe they can take what they want.