Well, my mother has asked me to digitize her collection too and have me host it. Originally, fine, you give your movies to me, I host them, same thing.
Did your mom buy your computer and hard drives? I doubt it. You spent your own money, right? So she’s giving you a whole bunch of stuff which is consuming your space. Quote out the cost of buying components for a separate server for her with her own drives. When she buys the parts, build her her own server and put her stuff on it.
You could accomplish what you’re trying by putting the GPU in a second computer. Further, most UPSes have a data interface, so that you could have the GPU computer plugged into the UPS too, but receive the signal when power is out, so it can save its work and shutdown quickly preserving power in the UPS batteries. The only concern there would be the max current output of the UPS in the event of a power outage being able to power both computers for a short time.