Is there anything else I could use, like an orange liqueur, or would it be too sweet?

If you don't have or don't want to use a bittersweet aperitif, I would go for an unsweetened or low sugar spirit like kirsch (cherry), slivovitz (plum) or tequila (cactus).
Even a brown spirit like rum or bourbon would taste interesting with the olive oil and other cake flavors.
But yes stay away from liqueurs, which will be too sweet and reduce the flavor contrast in the resulting cake.