The sidebar entry is still coming from mDNS. Play with the NAS’s config until it shows up the same way Windows peers with shares do: as an all-uppercase-named machine only visible under the WINS workgroup name in Networks. (Finder will give it a sidebar entry from then on after the first time you connect to it, IIRC.)
Also, wait 15 minutes. Because mDNS.
So basically I cannot have both Time Machine and SMB from the same NAS box servicing a Catalina Mac? I just tried disabling Time Machine share in Samba and Catalina Mac still complains "The operation can't be completed because the original item for "Share" can't be found." if I click the sidebar to connect to a share.