Let's take a look at the Cardassian Union from a lore perspective for a moment. As of 2385, they can no longer field an active military and are limited to a small planetary defense force. In exchange, the Federation provides them with material and humanitarian aid. Starfleet is largely responsible for the security of Cardassian space. Realistically speaking, the Union is essentially a glorified vassal of the Federation as it is. Why not just merge them into the Federation? At least then, you have a half-baked excuse for the Galors serving in Starfleet while simultaneously laying the ground work for the story of a True Way-based Cardassian faction. You even gain a story-based excuse to charge for Cardassians as a playable race.