This strategy may fail insofar as you may acquire in the course of installing Okular some of the second third order deps which by default in most systems won't be removed even if you remove Okular itself. You would then conclude erroneously that you had to install fewer extra things because you already have it.

It's a better bet to use your package manager for effectively a graph of dependencies ideally limited to a finite depth for brevity.

Looks like okular <- purpose <- kaccounts stuff also

okular <- kio <- half the kde universe

Regarding purpose

This framework offers the possibility to create integrate services and actions on any application without having to implement them specifically. Purpose will offer them mechanisms to list the different alternatives to execute given the requested action type and will facilitate components so that all the plugins can receive all the information they need.

Sounds sort of like android intents?

The situation remains that basically the first KDE app if you have nothing KDE/QT is 2GB the second and subsequent are 10MB. Honestly with storage costing almost nothing per GB and very limited developer resources this isn't a terrible design. Spending something you don't have in order to save people are resource none of them are going to care about doesn't seem like a great investment.