VPN tunnel between 2 routers

From your diagrams OPENVPN would be the option i would advise you to pursue. The reason why is that most SOHO and x86 wireless routers could be flashed with a WRT firmware which have built in VPN support. What i would recommend is a dedicated VPN server at your HQ as a router would experience a performance drop when you multiple simultaneous connections are established to the host server at your HQ. A custom firmware can be built easily that would give you out of the box configuration for customers who already have supported hardware. This can also work in your favor as you could find the hardware which works best for your needs and have these deployed with your custom firmware giving your customers true plug-n-play. I am fully capable of setting up your VPN and also building the custom firmware for your hardware.

Dear, I setup many VPN connections and have a lot of experience how configure every aspect of it looking forward to discuss it more Regards

Hello. My name is Stas. I work as a network and voip engineer for more than 5 years. I am MTCRE/MTCWE and CCNP certified engineer. I have worked with different companies and Internet Service Providers outsourcing. I have a very good experience in designing, setting up and maintaining network and voip solutions. Experienced in mikrotik, Cisco, Ubiquiti, FreePBX. I am novice on this site, but I have a very good experience. Please leave me a message if you are interested in. And give me more details on this project. I hope we'll have a good partnership. Thank you