It depends on what you need to do, basically you need to set the br0 of your hosts with the default neworking config of your servers and as @wusel suggests you can use clusters to split single hosts in order to have consistent networking configuration based on each additional IP you can purchase on your hetzner hosts.
I know it may not reply to your question, but a warm suggestion… you may have better results using OVH dedicated hardware, where you can have a vRack (transparent private networking https://www.ovh.com/us/solutions/vrack/), there you’ll set br0 on the vRack interface (usually eth1), and configure the gateway access and default server ip on the public interface (usually eth0).
Then you can purchase IP address blocks and link them to the vRack itself (so not linked to the single server, but your whole infrastructure)… in this way you can live migrate VMs.
I’m running NodeWeaver (OpenNebula Hyperconverged Infrastructure product) clusters for years in OVH and used both dummy (br0 networking) and openvswitch networking w/o any issue.
note: I’m not related to OVH in any way, but I’ve also started using hetzner and faced a lot of difficulties in using the network the way I wanted, so I switched to OVH.