IMO pi is perfect fit for pihole (low power and silent), but since its mission critical you want 2 devices running pihole. If one is down the other one will still be available. I have one pihole running on the server that runs all other services (including home assistant), and 2nd pihole on rpi4. Then I have another pi4 for 3D printer

Running pohole and homeassistant on the same device is not a problem for sure, but not sure about octoprint since it is using USB serial connection while printer is active (can be 12h+). Maybe it can work, but I wouldnt use 3D printer computer for anything else