That's a really hard problem to solve. But my advice, and what was working for me last time I tried, was using the Pi-hole as an exit node and forcing as much traffic through it as possible. The only downside to this is that your Pi-hole dashboard will be less interesting because your devices will show as the Pi-hole itself. If you're already doing that, then my only guess is getting obsessive about domain blocking until it stops, and even that will have its limits.