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 "spies" and retake Founding Titan "quaintly".
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)
Its dead simple, Karl Fritz could have brought the rumbling all over the world, but
He always hated the war.
That's why he vowed not have a war again (vow of renouncing war), also he vowed for whoever with royal blood have the founding titan will bound to this agreement that they will never wage a war in their lifetime. Also the vow states that only people with royal blood can use the absolute powers of Founding titan.
Here's a solid evidence, even if its a spoiler you can view this because it's not a major one:
And this has been always known by the rest of the world that, Fritz family will never wage a war against the world. This was the only reason that the Marleyans were never really afraid of attacking the Paradis Island, even though they were aware of the Founding titan's power.
So basically they just wanted the founding titan back (Reiss family wasn't really using it) so that they can use the Founding titan's power to gain control over other territories, because other territories were advancing in terms of weapons to take down the titans (like Beast and Armored), this can be confirmed from the starting episodes from the final season, where an armored train literally overpowered the Armored Titan, also later Armored titan helps Beast titan to destroy the ships, which first attacked Beast Titan (If armored titan wasn't there Beast titan would've fallen down.)
But later when the Marleyans knows the truth that founding titan was stolen from the Reiss family (Formerly known as Fritz family), the Marleyans started afraiding that the rumbling will happen sooner or later because the vow was not bounded to the person who doesn't belong to Fritz family. But also to start the rumbling founding titan holder needs royal blood. That's why they held the festival (which can be seen in the latest episode released on 28th Dec), so that they can regroup the other countries and attack on the Paradis Island before people from Paradis island start the rumbling.
Here's the major spoiler from manga, which justifies first statement:
When Zeke and Eren works together, Zeke wanted to command Ymir, so that the people from paradise island can never have a child. So Zeke needs his help as Eren cannot use founding titan power unless he touches someone with royal blood. Zeke wants his cooperation to implement his plans. So, Zeke describes about the vow to Eren. See how a person with royal blood is chained down so that vow can be maintained.