[help for design] Multi site with OSPF, VPN, Backup

Hello everyone
I have to modify a multi-site network to connect in FH in line architecture, and some remote site without FH;

I have a main site that must be able to communicate with all the sites.

I need to set up a network that in the case of a loss of FH have a backup route via VPN to connect to the main site.

also the remote site must be connected by VPN to the main site, in the case where the internet link of the main site falls I wish that there is a back up with an internet link of another site which is to connect in FH with the main site.

for maintenance a portabel PC must be able to connect to networks via OpenVPN.

for my part I can do the config Speedfusion, Pepvpn, and openvpn but I have doubts about the feasibility of the routing rules between the FH and VPN.

I thought of using OSPF but it is not my strong point.

If someone already has an architecture of type I would like to know the possibilities and the problem or encounter limits.

thank you
Dessin1.pdf (153.2 KB)