So, I kind of feel bad that nobody has yet replied to your question. And I am personally not best equipped to give you a comprehensive answer to that. But since none of our fantastic Solus team members have replied, let me say this.
First, I found this thread on our forum. Check out what Harvey writes there, he is a Team member and knows what he is talking about.
It seems to be, that eopkg, the current Solus package manager, does not delete downloaded packages on it's own, manually executing sudo eopkg dc would.
However, the Solus Software Center (graphical interface) would trim down cache from time to time.
As of "saving old packages", it doesn't make that much sense per se, since Solus is a rolling release which gets updated each friday. So, as you have mentioned, if you don't update for 3 week you might end up wit 2 GB of updates. So, old packages don't have any value for you.
But if I get the question right you have multiple devices and want tu update one, transfer the packages to the other machine and update there too, without re-downloading, that should be possible, if my understanding is correct.
At least, if you run the update in terminal with sudo eopkg up instead of software center.
But on the other hand, I don't have any comprehension of the internal processes of the eopkg. So, I don't know if when it tries to update if it's just searching /var/cache/eopkg/packages for the required packages and downloads them if they are not in there, or if it relies on a kind of database which knows of downloaded packages and just checks the database without checking the directory itself.
One way to figure that out could be to install a programm that you haven't had installed before(so it wouldn't be in any kind of downlaoded packages database, if it even exists), just a tiny one without many dependencies. simple-scan for example (if you don't already have installed it). But before installing it in terminal with sudo eopkg it simple-scan, you download the latest version of it from the repo, place that file in /var/cache/eopkg/packages, cut internet connection so that it can't download it on it's own, run sudo eopkg it simple-scan and see if it get's installed from /var/cache/eopkg/packages.
If that works, you should be able to apply this method for cross-device updates. (well, I mean not downloading packages manually from the repo, but just copy files from /var/cache/eopkg/packages)
I also have to mention that eopkg will be replaced by another package manager called moss, eventually, when Solus will be rebased on Serpent OS tooling.
What will or will not be possible with that package manager, I have no clue about.
