We need to secure at least a portion of the lowlands. Currently, the Highland Kingdom's borders go right up to the Great River that cuts down the western side of the lowlands. We 100% want to secure both banks of the river for when we build the lowland canal. Having such a vital, major supply route be vulnerable to a chronic, backstabbing enemy is not an option.
Whenever we next face a crisis, the Highland Kingdom is going to stab us again. If they can turn that stab into disruptions along one of our major internal trade routes, that's going to cause a lot of problems. Imagine the crisis caused in Valleyhome, Lower Valleyhome and Valleyguard once food shipments start becoming disrupted. If those three cities become higher level cities (which is extremely likely) after gaining access to the lowland's bulk food, we will be in for a world of hurt. The Stab loss and urban panic rolls could likely do us a lot of harm, even if trade is only briefly disrupted, and that's likely to happen on the back of another crisis.
Personally, I want to seize all of the lowlands area they hold and then turn the area nestled into the fork of the Great River all the way up to the Core into a March so that the Highland Kingdom won't be able to stab us again. Txolla can have virtually everything else in the lowlands. Hopefully we'll be able to contain their crazy until they collapse or we get big enough that we can tank the hit caused by breaking out the bendy straw during the next major environmental disruption. Cutting them off from the lowlands would also make the Highland Kingdom more vulnerable to developing problems due to lack of room to expand or generate Econ. It might also force them to devolve to being a minor power over time.
Once the Highland Kingdom is finally done, we can propagate the March south to get closer to the Not!Indian ocean. While the Harmurri have dominated the area of the lowlands between the two rivers, I'm not sure that their reach extends into the hills along the edges.