Which VPN to choose to combine branch networks?

Question

VPN

It is necessary to unite networks of branches by means of VPN. In general, the task is not for real life but for a course student. It is assumed that there are 10 branches, some behind NAT, and 5-10 will be added per year.
The priority is scalability and fault tolerance, then security.
Personally, I'm only familiar with OpenVPN. I like everything, but:
1. If the server is not available, the connection between clients breaks.
2. It is required to manually add routes, albeit in the server config, it's not convenient anyway, I want the routing to be dynamic.

Answer 1

L2TP+IPSec+OSPF

Answer 2

GRE over IPSec + OSPF