Yeah, took me a fair bit of googling for answers and getting conflicting things. Just made networkmanager autoconnect and set the priority of the wifi. You try to get it to do a virtual device but since I have a USB Wifi I have used that + the RPI's wifi too.
Anyway I made sure I documented my setup and have all my config on my github page, it is for Manjaro ARM but they're a systemd + network manager system it might be helpful in some parts for anyone who is configuring their own system (other than installing packages) github.com/wgauvin/astrogruff-setup
For Ubuntu, the proper way to do it is to use the NetworkManager files in /etc/netplan. You write up a procedure file (extension - .yaml) that contains the definitions for the etho and wlan settings. This file is VERY indent specific and you need to do a bit of research or find a good example of what works. I set up a static etho ip and a named Hotspot (set the SSID and Password) in this file. Everything comes up at startup, once I got the syntax all correct.
I just started looking at the Astroberry and after reading your thread this makes a lot of sense to move towards a 64 BIT OS. I'm sure after I see my first crash in 32bit, I'll be back here installing ubuntu. As we all know AP is so fragile and taking out the OS as a problem is a huge help. Thanks for posting this.