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
Not OP but loss of the Pi results in loss of network connectivity. A headache if you're home and never doing anything time-critical on the network. A disaster if you or anyone else is dependent on the network for anything time-sensitive (virtual doctors appointment, work call, etc), or you're away from home and unable to directly VPN to your router to reconfigure DNS settings.
As some others mentioned, when the DNS goes down (which pihole is) your whole network is down. With the fragility (and slowness) of the PI, it'd be more likely it will go down, sooner than later.
Considering the cost, a good alternative, imho, would be some sort of thin client, with an energy efficient CPU. So, instead of getting 2-3 PIs, better get one of these TCs, while keeping your PI as a DNS backup solution.
You could say that with most any hardware. You could have a dedicated server fail and have your network be down till you got another instance up or have a "fall back" kick in.
What makes Rasp Pi so much more unstable? If anything a couple PIs are definitely a cheaper solution compared to other hardware like you are suggesting?
Are they thou? In my region the 4Bs are selling at around 60 bucks (no case, no SD)... A "couple" of them (including some for backup and HA and Octoprint) would mean at least 4 of them, totalling at 240 bucks (or 300 with SD). For that money, one could get two (or even three) more-than-capable thin clients.