The rule that is being applied here is that the subject of that clause is "mastering literature", which is singular, so the verb is "was".
The phrase ", as well as sculpting, painting puppets and making costumes," is an additional phrase but not part of the subject. It would look more obvious if written as "...mastering literature (as well as sculpting, painting puppets and making costumes) was...". If you remove the part in brackets, the sentence should remain grammatically correct.
But I would tend to use "were" in this situation, treating the subject as the plural phrase. This is an example of when the subject gets separated from the verb, the verb form tends to adapt to the words that are close to it.
"Which was" and "which were" don't work; they would leave the sentence having no main clause, which is ungrammatical.