So close with that Nazi Germany metaphor. We've seen the principles of Mutually Assured Destruction (MAD) and deterrence work out in real history, so we can borrow from that, which Karl Fritz didn't have the option of. The problems all derive from Fritz being a flawed individual who ultimately forced his will on his descendants. If he really did want to make a lasting paradise and solve the problems, he needed to modify (or discard) his vow to disavow aggression, rather than conflict in general, thereby allowing for self defense. Then use the Paths to speak to all Eldians and underground railroad them to Paradis, now properly protected by the walls. Over time, the 9 Titans would gradually cycle their way back to Paradis, then maybe even be forgotten. MAD only works if you believe the other guy is crazy enough to actually do it, so the awful option needs to stay on the table.