Like the OP, I too have struggled with getting my VM in Cockpit to work with bridged networking so the VM could access host shares via Samba.
This thread has been a very interesting read, and has inspired me to find a solution. I came across this very useful article (skip over the 'installing KVM' part at the beginning and go to 'setup bridge networking with KVM').
Much of it follows the same steps as the OP, but with much more detail and some additional steps (notably disabling netfilter which is apparently necessary). So I did all that and it worked perfectly on my OMV setup.
One thing to add, though: once you have your host-bridge showing up in Cockpit, go to your VM's networking settings and configure them as Bridge to LAN with source br0 and model virtio. This combination is the only one that works for me; neither Direct attachment nor Virtual network were correct.
Now OMV and my VM live on the same IP subnet and have their own IP address each, and they can talk to each other. Just for kicks, I opened Firefox within the VM and browsed to the OMV IP, and sure enough the web GUI (login page) showed up.
The only remaining issue for me is a purely cosmetic one: in the web GUI, the old configuration (only my physical network interface eno1) is still showing instead of the new config. This must have something to do with the files in /etc/netplan - I wonder if I should replace 20-openmediavault-eno1.yaml with the contents of my own 00-installer-config.yaml or simply delete it instead.
Wondering if someone could weigh in on this last point?