That’s what I do, but with mailbox.org instead of proton, to name an alternative. They even offer temporary random addresses with the click of a button.

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