You see this little planet? Is a planet roughly the size of earth, just slightly smaller circumference by 50 kilometers and a smaller gravity of 0.8g with little variations around the world, just like on earth. Apparently there is nothing too strange about this planet, for now....
Looking closer you can notice this unusual geography shaped as a lake of freshwater separated from the salt oceans by a structure of land barriers. This lake has also some large isles inside which suggest they could be separated continents. But, that's not the case. The entire land is on the same giant tectonic plate that occupies roughly half the surface of this planet, so it's on the same titanic continent.
What I want is to achieve a perfect stability for the land continent, the oceans could be as chaotic as possible, I don't care. I just want the shape of the land to remain unchanged until this planet gets either destroyed by the end of the universe or by a sun and planetary collisions. Anyway, again the giant lake has to remain eternally unchanged, the lake and sea have also to never meet directly.
So how would that world achieve this shape permanently in a natural way?
♦If needed the giant land-lake tectonic plate can move around the world but not change shape.