You can simply have a planet where the water level did raise at one point in history and only mountain chains are above the water level, noone limits the amount of mountains there could be in a world. And while mountains can be created by movement, volcanoes can also create rather decent mountains so one could think the planet was completely submerged in the past and eventually eruptions created the existing mountains, this could be volcanoes not active anymore so no ash clouds, volcanic ground is also very fertile. As for the agriculture there’s plenty of examples of terrace cultivation in our world, be it for crops needing a lot of water like rice or not. In the Andes Incas used the system a lot
