Networking isn’t my strong point, but my impression is that if you do this (e.g. with a Pi-hole) there isn’t a way to gracefully failover all your in-home devices to the second hop when your Pi-hole is down. In practice this means that to avoid introducing an extra point of failure you can only really do this with an appropriate router, which pigeonholes your router selection if your preferred router isn’t one that already supported this.