What are the benefit for user of a VPN with a DNS leak ?
I can’t tell because I don’t see one DNS leak, either on the client sent to the VPN or the one excluded. Both use CloudFlare DoT through the VPN (which isn’t optimal but cannot be called leak).
What are the benefit of dont useing existing wifi hardware which have open source driver ?
Having better speed with WIfi ac (because there is no open source wifi ac or ax).
What are the benefit for user of a router firmware which have a closed source part ?
see above : faster wifi and functions not available as open source like trend micro on ASUSWRT.
What are the benefit of a DNS leaky router based solution against only installing a VPN software on the PC which need a VPN ?
Not all devices behind a router (and I don’t see it leaking in the case of my MV1000) are PCs. Once properly configured in the router, I can chose which device goes in the VPN or not without having to install anything on each and every device (when ever a VPN can possibly be installed, try that in a cloud camera for example).