As someone running a pihole off an old radio 2B, your MacBook will be more than sufficient to run what is needed. My only advice would be to get an Ethernet adapter if that model doesn't have one. Losing valid dns queries due to wifi packet loss would be annoying. Beyond that, just google a guide and go, it's super straightforward to set up and manage.

That'd be fine. Laptop cooling fan might die from being on all the time as well as mechanical drive issues at that age but it's solid hardware otherwise. Pihole is not overly intensive. Ideally make sure the pihole machine is on a wired network connection inside your LAN, because wifi routing latency will be bad otherwise. So that may necessitate a thunderbolt ethernet adapter, but I've bodged together much worse before lol.

Ubuntu (or Canonical, their parent company) has gotten more pushy with their paid service. Personally for me, I'm moving off of Ubuntu to Debian pure systems or Arch because when I ssh to my Ubuntu file server, the MOTD tells me I can pay for some kind of premium service and get 35 additional security updates. So, that's it. That's my line in the sand. Don't advertise to me on my terminal

(And then there's all the shit about Snap being installed by default, and I'm just at a point where I only want installed what I want installed, etc)

But you do you man. If Ubuntu works great for you, stick with it. You may change your mind later down the road, you may not. As long as you're happy with it right now that all that matters.