All makes sense now. If you don't intend using the Archer for wifi, it might be worth considering a cheaper model as most of the benefit of the more expensive models is in improved wifi speed.
AIUI, the Google wifi things are only useable in 'mesh' mode when the master google thing is effectively the router. If you use it in Bridge mode, which you have to do if you are connecting it to your router and replacing the routers wifi, then MESH won't work ... that's what I read into the pretty rubbish instructions for the thing.
Just when I thought I was getting somewhere, maybe I'll try the BT mesh offering, the press seem to like them.
I said I would never buy BT hardware again too.
You could always get an Openreach modem rather than the archer and use the Google wifi as your router and wifi.
Scrub that, just seen the reason for the Archer.