OK I did a bit more digging, looks like what I want is essentially this part in the paid plans: "Restrict internet access to specific allow-listed domains for

OK I did a bit more digging, looks like what I want is essentially this part in the paid plans: "Restrict internet access to specific allow-listed domains for a "locked-down" environment" Naturally I'll probably try with Pis, and if that doesn't work very well I'll buy a plan. $19.95/year doesn't sound bad to me.