My home-assistant installation alone is too much for my Raspberry Pi 3. It depends entirely on how much data it’s processing and needing to keep in memory.

Octoprint needs to respond in a timely manner, so you will want to have the system mostly idle (at least below 60 percent CPU at all times), preferably octoprint should be the only thing running on the system unless it’s rather powerful.

If I were you, I would install octoprint exclusively on your Raspberry Pi 3, and then buy a Raspberry Pi 4 for the other services.

I’m running Pi-hole and a wireguard VPN on an old Raspberry Pi 2, which is perfectly fine if you are not expecting gigabit speeds on the VPN.