Success! We’ve managed to spend a couple of nights in the van, it all works well, Alexa for the music and the Roku / Plex combo. Other than a couple of issues, below, it’s worked flawlessly and has added benefit without being intrusive. I’ve added some more flic buttons and tweaked some plex scripts (and a UPS buzzer fix), but for actually using the system it has performed really well!
Issues… so there have been a couple of teething problems. The main one being that Alexa frequently loses connection to Kasa, but the Kasa app works fine, a weird one that I haven’t managed to get to the bottom of. I have put in a few more flic buttons to work round it, but it’s still an ongoing issue at the moment. The bandwidth at the van seems to fluctuate a lot (see below), I am intending to replace the GL-Inet device in time, but I am going to wait for an ARM version of OPNSense and will put in another pi for the firewall / VPN attached to some form of 4G or Starlink router. Finally the wireless seems to drop sometimes, I think this might be due to the VPN connection, but I am going to add a TP Link access point to see if it makes any difference. It all works well enough for now though and we’ve spent a lovely couple of weekends there!