I need to connect several devices to the internet via WiFi but keep them away from the main LAN. Ideally this would be a simple matter of using the "Guest Network" access that many routers provide

Do the 2763/2765 series offer a guest network via the built in WiFi (not via a separate AP), or do I need to set up separate VLANS? Preferably it would simply be "guest network" in the WiFi settings, but I suspect I will need to go the long route.

Any advice appreciated.

To set this up properly, create a new vLAN for Guest access in DrayOS (preferable non-routable to your trusted LAN) and create a new WLAN that puts clients on this new Guest vLAN. Apply a strong WPA2 to better password, then create a QRCODE of the WLAN credentials which you can share with Guests to join on demand. Anyone who joins that Guest network will be isolated from your Trusted LAN.

That's the basics, there are more advanced things you can do like selective isolation using Firewall rules, Hotspot with authentication and automated changing of the Guess WLAN password so they have to rejoin after say a week of access but that's enough to get you going.

Sorry for being a bit late back to this but thanks very much for the help. What I want does appear doable without too much faff.

Is the current model the 2763 or the 2765? The both appear to have the same spec, the only difference with the 2766 is Gfast, which I don't need.