I am still trying to figure out if you have a specific setup in mind, I recommend using Proxmox as my os and having Home Assistant on proxmox with whatever other os you want(this will turn the main computer into a server terminal and you’ll need another computer and browser to access it).
This thread can explain a little better using mac os but I think they still use a VM.
Otherwise Is there a specific thing your after with this install? Do you still want the computer to run macos? Do you want to learn Ubuntu? Do you want Home Assistant to be the OS on the mac?
My proxmox is on a system meant only for it and my home assistant, it is never used as a normal computer to prevent anything from happening to my home assistant.
Also I think you can boot Linux from USB and use Balena on USB linux to write the SSD.