I installed the repair system in one of the mounts on my first ship, then upgraded to a different ship, and now I cannot ever use repair systems again apparently? You can replace a thing that is installed in your current ship's weapon slots, but there's no way to uninstall from the collection screen and no way to switch back to a lower level ship.
Without a way to repair, and with shields apparently doing nothing, eventually it doesn't matter how much firepower you have because each enemy unit will get one shot off before it dies and your hull is inevitably eroded away.
Also many weapons have damage numbers change by a different value from what is displayed before you buy the upgrade, which is confusing.