In case anyone has a similar problem, the plugin depends on the wireguard meta package and on a 6.x kernel and headers (at least the >5.5 kernels I tried were unstable except the 6.1.11).
So unfreeze in armbian-config and select the 6.1.11 kernel, apt install linux-headers-edge-rockchip64 to install the headers then manually install the wireguard meta package then OMV from there.
Thanks for your time Igor.