Is there a way to link to an org (sub)heading in another node?
Yes and no.
Org-roam does not add to Org’s link syntax. ID links in Org mode only link to an ID; there is no syntactical way to link to an ID’s (sub)heading. So… This is not correct Org’s link.
You can use Org’s file link synax like these, but I don’t think these are not recognized as backlinks by Org-roam – you can try:
‘file:projects.org::*task title’ (headline search) ‘file:projects.org::#custom-id’ (headline search)