Wireguard in a box?

Hey all, trying to figure something out to see if this is possible, and appreciate any advice or pointers!

I'm looking for a box of some sort that i can give to a family member, D.

D has a normal ISP router and isn't interested in me setting up a different router for them as i'd have to support it too (and doesn't want to have to reconnect all their wifi devices!)

Any case here is what i'm trying to solve. There are a couple of devices in the house which we need to have all internet bound traffic routed through wireguard to my wireguard server. Importantly, D wants to still be able to access those devices locally, for example a google home/alexa speaker, D wants to be able to cast to them from any other device on her network, whether they are using the wireguard box or just directly connected to her router.

So, i guess what i am looking for is a box which can act as a switch

• devices connected via the box get an IP from D's ISP router (DHCP) and live on the same network
• devices connected via the box are able to send/receive broadcasts, mDNS works, and direct connections with any other device on the ISP router
• but any internet facing traffic from devices connected via the box is sent via wireguard

Wondering if anyone has seen or build something like this? Thanks again