Easier, safer, and faster births
Operation Reboot Reproduction
One of the unfortunate things about how humans evolved is that our reproduction systems kind of suck. Infant mortality is a real problem (in some countries like Sierra Leone it can be almost as high as 10%) and even in first world countries, it’s unpleasant and painful.
Similarly unfortunate are the biological side effects of our reproductive cycle: menstrual products alone account for billions of dollars in expenses every year, and birth control methods, while extensive and varied, are often unpleasant and can have side effects. So let’s fix this.
...via artificial wombs
Extend the role of the symbiote from being merely inside its hosts to having an outside form as well. Fungal structures, ranging in various sizes, that serve as incubators for human children. Best of all, to start it up, the two lucky parents just have to get close: the sporogenesis process wraps their genetic material, and transmits to the incubator in little puffs of white smoke.
Meanwhile, the human reproductive organs are sterilized and altered in a way that removes the inconvenient side effects of having it, while maintaining the aspects that humans most enjoy.
Your symbiotic species can outreproduce its foes without ever needing to put any of its member or out of commission for that purpose.