It sounds like the gp’s objection might be that they’d like for their children to use iPhone and its parental controls, but they themselves would prefer to use other platforms to manage the supervision features. As far as I know, you do in fact have to be within the apple ecosystem to manage the ScreenTime features as you suggest.

Firewalla looks really thoughtfully designed! I’m glad to be aware of it.

I actually have Firewalla (it's great!). But it's only helpful when a kid's phone is connected to the network itself (which they can escape easily by disconnecting from wifi). The native stuff works on all networks.

I’ve been assessing this. Is it possible to MDM-install a VPN to the home network so the same policies are enforced?

I’ve always thought their implementation of this is pretty poor. Lots of granular options but it’s difficult to cover all bases.