The steppe lancer is a unit which has seen very little use recently despite being available to several popular civs such as Tatars and Mongols. This is because it has no defined niche. There are no situations where you would use the steppe lancer instead of the knight or the light cav line. It’s too expensive to be light cav and too weak to be a knight. Something needs to change. There are, in my opinion, several ways to do things.
Make it similar to the light cav. Cheapen its gold cost a bit and adjust its stats slightly, maybe reducing the attack reload time a little bit.
Make it similar to the knight line. Keep the gold cost as it is and increase its stats to make it a weaker knight but with the extra range and slightly higher mobility.
This idea is the one I like the most. Turn it into a new type of unit: shock cavalry. The way I see this, and the way I would also prefer the coustillier to be, is a unit with high attack but also high reload time or low health, making it weak in a head on fight but strong when microed without having a weird and buggy charge mechanic. This would mean you could use a group of them to quickly snipe siege or monks or even archers before you attack with your main force. Obviously if the unit does end up this way it won’t make up the main part of an army composition, just a small supporting part.