I also found out the OSPF issue is due to the implementation in the firmware: OSPF is not supported on Virtuals WANs. Swapping the virtual WAN with a real one solves the problem: OSPF does a nice job now. OSPF over Virtual WAN would be a nice feature request.
The only thing to figure out now is the handover between (Rajant) Mesh and Speedfusion (over 4G/5G). Both are different OSPF areas where Outbound policies prioritize the Mesh interface. Only when the OSPF times out and the route is lost, traffic will go over the Speedfusion tunnel. This can cause the device to stick on a poor quality mesh link, or cause a certain time out period before re-routing.
I would rather prefer some kind of Speedfusion tunnel between two Peps, bounding the mesh network and the 4G connection. However, as the 4G of both devices is not always available and certainly not on a public IP address (but behind CG-NAT) this would cause the need for another publicly available device, e.g. a Speedfusion Hub on some cloud provider. In that case, all Speedfusion traffic, even when the Speedfusion tunnel over the Rajant link is prioritized, would go back and forward over the public internet, which is an unnecessary delay.
Better suggestions are welcome.