As I said, I don't see myself fighting against Finland. I can always reason with decent liberal democracy and probably even convert them. Finland is effectively anarchic state - socialist, but you can trade all you want if you want; regulations are minimal and made by actual people, taxes are high (brutally progressive) but you know they are used for the community.

As you can see from modern conflicts, DMZ and stuff matter shit if your opponent is deragned maniac like Putin or Tump, and any ceasefire would be just them regrouping for final attack. They matter little if your opponents are "regular leaders" under pressure of international relations. Border rules are enforced at please of each side, and you'll need to force your opponent.

They'll need guaranties as well by the way. They sure are proposing something in their offer - if you are overgrown revolt, most of your forces might be guerilla (technically all they are non-coms and criminals by international law until you are recognized as a state). Removing your troops from buffer zone pretty much banning entry altogether or it means nothing. If you are not allowed to control execution of treaty, the buffer zone will be militarizred in 20 minutes.

And this Finland reference shows one more thing - the only guaranties in modern world, apart from nukes, are international alliances. You could be winning, but as long as the rule of triangle is in force globally, things can change, and without an alliance (real one, where you interdepend) you'll be crushed soon. So - just look at the map and see - make your state useful for someone you are not fighting against, produce something everyone needs, and be located where force needs to be projected - and you'll be reasonably safe. Do less - and you'll be eternal proxy war playground. Do nothing - and you will not be.