Good morning,
router is in production and yes it works like a charm! Not only the multi-wan routing works with stickiness but also the network itself seems to be much quicker than with the old Zentyal setup.
LBNL I am also happy about the network configuration options (I need some static routes to connect to some legacy network, Zentyal required some manual hacks to achieve what I needed, nethserver just works)
So thanks from a happy migrator!