Not sure whether this is helpful, but I had a similar problem when setting up new VPS instances for Vultr. The default cloud.ini configuration for Ubuntu servers on Vultr runs "apt-get update && apt-get upgrade" on the first boot. This was causing real problems with my setup scripts, which had trouble dealing with upgrades happening in the background. As a result, I looked all over the Internet to find a way to shut down automatic upgrades on first boot of my VPS (and ran into this page several times), but did not find anything directly on point.
Ultimately, I figured out that the "user-data" feature of cloud.ini can be used to override the cloud.ini setup defaults, and I thought I would share my experience.
I set the contents of user-data to the following:
#cloud-config
package_update: false
package_upgrade: false
Vultr requires this to be passed to the Vultr API as a base64 one-liner, which comes out as follows:
I2Nsb3VkLWNvbmZpZwpwYWNrYWdlX3VwZGF0ZTogZmFsc2UKcGFja2FnZV91cGdyYWRlOiBmYWxzZQ==
This worked for setting up a new Ubuntu 20.04 VPS on Vultr. Vultr no longer automatically runs updates or upgrades on first boot (allowing me to fine-tune how this is handled in my setup process). For a stand-alone computer (e.g., a Raspberry Pi at home), it looks like the following website might explain how to inject a user-data file into a setup process that relies on an ISO: https://www.pugetsystems.com/labs/hpc/How-To-Make-Ubuntu-Autoinstall-ISO-with-Cloud-init-2213/. However, I have not actually tried that myself, so your mileage may vary.