I can add the light to the Tapo app ok - but it seems to indicate that once it is in Tapo it's just a matter of "Adding" it to Home - but I can't seem to find any way to do that. The Home App will...

I suppose that's technically true if all traffic is destined for the ISP via a single egress. If there were any P2P traffic a mesh on a LAN is conceivable. Thanks, I didn't know the X20 would daisy...

As has been noted by others in this forum - the Deco X20 forces you into a star network configuration with every satellite node being connected to whichever node you designated as Main. This may work...