Thanks guys, these are very useful methods and I'll mention these as alternatives to disk cleanups recommended at https://unix.stackexchange.com/q/774199/233262 (this would probably very useful to have at places about upgrades failing due to disk space issues even though people only look these up once the problems already occurred).
However, the problem of the upgrade requiring more disk space than displayed at first remains and the command by Zeimetz can't be used with a built-in rememberable well-known command like sudo apt-get upgrade --stepwise I don't think peripheral devices would be the common case for personal computers, rather one would simply specify a directory on a partition that is larger than the root partition. Upgrading individual packages is not what this is about in case that wasn't clear. It's one upgrade but it's separated into several steps where one batch of packages are download and installed, the cache deleted, and then the next batch. If the upgrade breaks in between due to disk space, because the user aborted it, a crash, or an error, then only some packages are upgraded...a stepwise upgrade could if anything be a way to *avoid* that (or at least interruptions due to disk space problems) and to make them less problematic. The key thing is that it would be usable with a simple command (such as by adding --stepwise)...if that command only executes a few already existing commands with no apt changes required for the basic functionality of this, that's all the better.
