You can do this without taking the SD card out while Ubuntu is running:
sudo vim /boot/firmware/usercfg.txt
Or open a terminal and type
sudo lightdm-settings
and go to the Users tab and fill out your username (ubuntu by default on Ubuntu Server but you can rename the user if you like and this also works on other versions of Ubuntu with lightdm installed).
Thanks Wouter,
I found out later that there are two usercfg.txt files, one in the system-boot partition (for modifying that you need to take out the SD card) and one in the boot/firmware folder. When I wrote up the instructions, I had not been aware that that one would also be used during boot up from the SD card.
Thanks also for pointing out the short-cut using the lightdm-settings command.
Overall, the setup is pretty painless. I still prefer the full MATE desktop, though, over just installing the core, as it contains a few more tweaks I like in MATE which I otherwise have to install separately, but that is a matter of individual taste.
Jo
Backgrounds, screensavers, panel items, various applications prepopulated in the menu I have gotten used to. Nothing you need to run Kstars.
I just like to set up Ubuntu on the Pi in much the same way as I have it on my desktop, saves me time navigating. At home, I am mostly using Ubuntu, I use my Mac almost exclusively for work (Word, Powerpoint, Excel, Endnote, Acrobat - none of the LibreOffice equivalents are compatible enough to port from Linux to Mac or Windows). One reason why I may have drifted that way is that my desktop has 32 GB of RAM and 8 cores, my Macbook Air only has 8Gb and it is getting a bit slow these days.
I had gone the route of minimal installation before and ended up spending a lot of time identifying the components I was missing and had gotten used to, then installing them one by one.
As I wrote, it is mainly a matter of personal taste. If you want to run the RPi4 lean and mean, that certainly should help its performance with Kstars, but I find that it is plenty powerful and performance does not suffer by installing these other tweaks, unless you are running memory intensive tasks like multiple Firefox windows etc. at the same time you are trying to run Kstars. That could be a problem. And with a large memory card, like a 128 Gb microSD card or with an external SSD connected for saving images, the 2 or 3 Gb of additional space it takes no longer is a factor.
Downloading the Ubuntu image and burning to SD is now much easier with the Raspberry Pi Imager. Download from www.raspberrypi.org/downloads/
I noticed that when I installed a desktop that the network manager icon on the taskbar doesn't work. I believe that is because by default Ubuntu 20.04 uses netplan and in turn that defaults to network. That can be changed in /etc/netplan/50-cloud-init.yaml I think. I haven't tested it with a desktop as I decided to go with a non-GUI setup but I did switch to NetworkManager on the server as I'm more familiar with it.