The present technology is directed to implantable medical devices for draining fluid from a first body region to a second body region. Some embodiments of the present technology provide adjustable devices that are selectively titratable to provide various levels of therapy. For example, the adjustable devices can have a drainage element with a lumen extending therethrough, a flow control element, and an actuation assembly. The actuation assembly can drive movement of the flow control element to change a dimension of and/or a flow resistance through the lumen, thereby increasing or decreasing the relative drainage rate of aqueous from an eye. In some embodiments, the actuation assembly and the flow control element together operate as a ratchet mechanism that can selectively move the flow control element between a plurality of positions and lock the device in a desired configuration until further actuation of the actuation assembly.