rise high, be well along
闌 centers on reaching a high point or advanced stage: rising high, being well along in progress, or approaching a peak.
闌 combines 門 (gate) with 柬, which likely contributes the sound and may relate to selecting or separating. The exact historical development is uncertain, but the character is associated with the idea of reaching a high or advanced state.
Inside a gate 門, the complex 柬 looks like a bundle of sticks rising high. Picture a gate with tall bundles stacked inside, reaching upward.
For ラン, imagine a runner reaching the high point of a race: run -> ラン, and the gate marks the finish line where they rise high.