It is possible that I am not looking at it correctly..., but systemd-resolved I think it is not here but it is in bullseye-backports, although only in the suggests category https://packages.debian.org/bullseye-backports/systemd
Oh yes, seems Debian has outsourced the binary into a separate package. This is maybe a problem that can not be solved by openmediavault because backports are not supported because of reasons like that. Thus it is not possible to add a package to the dependency list which does not exist in stable. Backports differ from stable and introduce new and unexpected things.