Why did the Marleyans go after the Founding Titan if they were aware of King Fritz' warning to organize the rumbling? The Marleyans were clearly after the founding titan to gain control of Paradis Island's resources and to finish off the Eldians for good. But why did they even plan on bothering them, knowing the King's warning of sending thousands of colossal titans to stomp the world? Geologically, fossil fuels could be found at so many places other than Paradis, and most of the Eldians were either prosecuted in Marley or sealed within the walls. Also, the King's will to renounce any sort of war was passed on to each of his generations. With everything going to the Marleyans' advantage, was the entire plan of breaching the walls worth it after all?
I believe Marleyans are in dire situation - theyve created a strong Empire, but other countries learned how to deal with their main weapon - titans :)
But with launching full-scale attack, they would risk the rumbling happening.
So their plans was to send &quot;spies&quot; and retake Founding Titan &quot;quaintly&quot;.
About the Founding Titan power itself, it needs royal blood as you mentioned. If someone with Royal Blood inherits Founding Titan, they become pacifist. But Marleyans are probably aware there's an other possibility.
Eren activated Fouding Titan by getting in contact with titan with Royal Blood. So he came to the realization that if Historia would become a Titan, she could be used to activate his Founding Power (apparently human form isnt enough, you need titan with royal blood).
Marleyans have a titan with royal blood. So if they capture Foudning Titan, they can give it to someone without royal blood.
Then Marleyans can use the titans inside the walls and many other assets. That would mean dominating entire world for good.
Anyway, i think that's you're right that breaching the wall was very risky. King Fritz could've start rumbling at this point, but maybe Marleyans assume he wont do it, because it's his last resort.
Also they've been playing it safe, sending mindless titans to Paradis for very long time and King did nothing to stop them. They probably realized King Fritz will try to avoid conflict and won't fight back.
Also lot of time have pass and i believe there's many doubts about what's about Paradis true and what's not.No one have seem an army of colosal titans, so maybe Marleyans started doubting it even exhists?
Its also not so rare for ruling class to do something reckless to keep the status quo x)