While religion provides a nice set of bonuses for a civilization, there isn't a lot rewards for spreading your religion to other territories (especially those of other empires.) Besides generating greivences and stability malus there isn't a lot you get from spreading your religion (given there are no era stars related to religious influence.) While there are a couple of civic choices that help it would be nice to have a few more tenants perhaps have less of an initial benefit, but a scaling benefit based on followers/territories that follow that religion. As much as I hate to use Civ 6 as an example the first belief you choose is that system is a "Founder Belief" that scales throughout the game if you choose to pursue a religious strategy.