It's been some time since I ran this myself but I used to have an openwrt router sit behind my ISP modem/router (it was one unit).

Multiple times I caught unexpected traffic and was able to block it piecewise including a lightbulb(!) that was sending mb packets of god knows what home.

If they were ethernet based I reckon some smart VLAN capable switches would be nice even at home to keep them out of each others business.

I moved home a few years ago and now I'm back with the ISP router and do often feel a little in the dark about what my network is doing!

https://openwrt.org/