Yes, the QCA9531 routers only have 16MB of space. But you can insert an old USB stick and increase the space with Luci> MountPoints and make / overlay. I have 2 AR750s with old 256MB USB sticks and I can install all the plugins I want with no problem.
EDIT: You can only install Luci when you update the firmware without saving anything or resetting to factory default because only 2mb free left and after Luci installed do / overlay in Luci> Mount Points and after reboot you will see that it works because it appears the password page asking you to set the router password as for the first time and then all plugins will be installed in the USB stick space.