An x86_64 board with AES-NI and at least two Ethernet phys, preferably Intel over Realtek. Minimum of dual core; additional cores are unlikely to further speed a single VPN connection (VPNs are typically single-threaded). If needed, an off-board, managed switch.
I assumed you meant "failover" rather than "load balance" for the two providers, based on the illustration. If you truly have both running simultaneously, then at least three cores would be recommended.
I conducted benchmark testing OpenVPN connection.
Connections speed to ISP on both sides 100Mbps
On server side real PC server.
For testing used IPerf 2.10
On client side:
I would agree that "cheap, Chinese" x86_64 boards are perhaps not the best choice for a critical piece of infrastructure. I presently use PC Engines APU2C4 and APU3C4 boards and find they have low power consumption (under 10 W running a desktop distro including dual Samsung SSDs under ZFS), reasonable performance, and very good reliability. Their processor, however, is not as powerful as other options available today.
The benchmarks that @oslyak provided are insightful. The FX-8320 is probably a good point of comparison, as isn't a "screamer", with single-core Passmark performance as one measure of around 1400. For comparison the AMD Ryzen 5 2600x has single-core Passmark of 2143.
I can't provide further recommendations as I don't own any of the newer, better quality, x86_64 boards. From what I have read, the WRT32X seems to be a solid unit. In Top ten routers currently in use? - #4 by slh, slh states
Both mvebu and x86_64 can be used for 1 GBit/s WAN speeds.
It all boils down to upstream support, Marvel Armada 38X have very good support although you might want to start looking at 64-bit ARM instead of 32-bit...
So the setup will be: Modem -> Mini PC -> WiFi Router
Right? But how can I whitelist some devices which should't go via the VPN?
When the VPN is runnig via Mini PC, the Mini PC can just the the WiFi Router?
Jalapeno Board would be able to do this and I think pretty easily. Built in hardware nat and hardware crypto. Just get a switch, not sure where you are located but monoprice is not bad for switches I use their 8 port gigabit and its solid to me.
As for load balancing VPN not sure on that I personally use Wireguard.
Presuming that by "router" you mean the "WiFi Router" you previously mentioned, then I believe that you need to do is use it as AP (and that's only if the Mini PC doesn't have suitable wireless card), so you basically set a static IP for the AP in the same subnet as the Main PC and connect it to the LAN port in the AP.