Please explain your feature request to the best of your abilities:

Add an option to allow the toggle of what town level is able to declare war with, basically making players strive to expand and grind in order to declare the wars they want to do.

This is actually already doable as of 0.100.2.0, with town and nation ranks locked behind town_level and nation_level.

You'd add a rank to your townyperms.yml, and set it up with a required level, and in that rank add the permission node to declare different war types. Then a mayor would need to grow their town, and then assign the rank to themselves in order to start the war.