I questioned that in my first response, but as I have not kept up on the subject (been using Time Capsule for a few years), I didn't go too deep into it.
les
If I am not mistaken, the Server app does enable a Mac to share an external drive to other Macs for TM backups. At least it did at one point, but as much as they are deprecating features, I wouldn't be surprised if it has been removed. No bets on future support as they seem to be dropping a lot of server functionality.
You don't necessarily need a Time Capsule. AP Extreme can share USB drives as a TM destination. The Extreme doesn't even need WiFi enabled, nor configured as a router. Since you already have a disk, you could find an Extreme on eBay for under $100 and use it as a TM "Server".
https://support.apple.com/en-us/HT202784
Apple discontinued allowing TM disks on Extremes prior to the AC model which is the tall "cylinder" form factor (like an extra tall AppleTV).
If you go this route, use Ethernet to connect to your router, turn off WiFi, and set Network Mode to Bridged.