Is spotube just another client for Spotify? Or what does it do? How is it getting rid of Spotify? You could host navidrome on your NAS and access it via webbrowser. If you don’t like the interface, there are several subsonic alternatives.

https://forums.lenovo.com/t5/ThinkCentre-A-E-M-S-Series/adding-hdd-to-M720s-power-connector/m-p/4377028

Seems to be the way, this thread on the Lenovo forum supports your idea.

There is also a guy on the german mydealz forum, who is running 2 HDDs on one power connector and it seems to work for him (account required to read): https://www.mydealz.de/deals/lenovo-thinkcentre-m720s-sff-office-pc-intel-i3-8100-8gb-ram-aufrustbar-256gb-ssd-window-11-pro-hdmi-usb-c-refurbished-office-pc-2535136

Every night my NAS wakes up, fetches the data from my unraid server, backs it up and also sends an encrypted backup to a hetzner storage box on a server in a different country. Haven’t had to use it yet, but in theory works fine. Should do some more regular testing though…