The 30 days limit is just fine.
The policy of which you want to take a huge nation like US with about 2500 citizens right now to a few hundreds is unrealistic. More will always join. The new recruits for US can be anything from 200-300 players to 800-900 players per month. Are you going to keep booting even 200 players per month? What will be the next step? To ask for a mass removal tool?
That's too much in my opinion. If you want a small and tidy nation, pick one that gets few recruits. Nations like the US, England, etc get too many new players per month to operate them in such a way. In my opinion, they need a completely different way of operation than the one you want to apply.
Of course, I am not going to interfere and tell you how to operate it. I am not going to approve a tweak that will not work either, like the one you suggest. Even if we removed the inactivity limit and you could boot anyone, you would still not be able to boot them all due to the sheer numbers of new recruits every month.
Not to mention that the majority of true inactive players are removed anyway by the game.