So, I looked into the concept of if I could use some kind of launch option command in Steam to make the game run on all cores, and learned that when one program launches another, it's affinity settings carry over. I ended up backtracking and finding that the FF14 launcher is also running on just cores 7 and 8, and so is steamcmd.exe. So I tried manually changing steamcmd.exe to run on all cores, and now FF14 does too, at least until I rebooted my computer. Now I don't even see steamcmd.exe under task manager, everything else related to Steam that is there is running on all cores, and the FF14 launcher still launches on just cores 7 and 8. I'm not sure if I have a Steam problem or what to do now.