I am not a great believer in long term happiness. I studied under a wise Zen master for two years that said the following:
Happiness and sadness are fleeting emotions. We are only expected to experience them for snapshots in time. A sense of peace and acceptance can, however, be achievable.
Not only is a DIY UPS unreliable, but it could be potentially dangerous unless you are an electrician, an electrical engineer, or somebody that has extensive knowledge of both the engineering side and safety side. How many CyberPower units do you have? It's impressive that you have enough UPS power to run a NAS for that long.