I dont think a shared key setup is easier to manage with 50+ different tunnels.
In a shared key setup you dont use pushes on the server to add routes to the clients.
You have to add the routes in the client config directly.
Meaning if you ever add a new office you will have to change the configuration of every client.If you use a PKI you just add a push command on the server and reinitialize the connections.
If you want to use pushes you have to use a PKI.
What about starting 50 instances of openvpn with shared key. Is it considerable load for the system or there is no real way to tell?
I will look into the PKI setup.