I have a Librem 13v3. I never had to install ‘aptitude’ itself.
I use:
sudo apt update
sudo apt full-upgrade
To my memory, the only time I had difficulty updating was with a sailing app I use for navigation that is not in the PureOS repository. It needed curl4 that was not ready in Purism at that time. I have seen others with conflicts.
PureOS is a rolling release. Recently, that has been discussed here.