What you took and the time you took it is besides the point. Your concern is between the tense of the main clause and the tense of the final clause, the verb tense in the adjective clause “(that) I took away from my work experience” is irrelevant.
The most important thing was/is
that a great teacher is compassionate
You want to show same time action between the main and dependent:
CORRECT: "The most important thing I took away from my work experience is that a great teacher is compassionate.”
The fact “is” is the verb for the teacher, makes it a fact that the teacher was and is and probably will always be a compassionate teacher. The verb “is” is Present Tense used to state facts or show habitual action.
“thing” is / teacher is compassionate [thing = fact AND teacher compassionate = fact]
If you really want to get grammatical, I'll show you that not only is the verb "is" Present Tense but also that it is a linking verb that links the subject to the noun or adjective on the other side of the verb, and in your sentence, that would be the noun clause "that a great teacher is compassionate" which serves the purpose of being a noun in the sentence, known as the predicate nominative."
I have sources to prove everything I am saying to you.
You can leave out the modifier. After all it is only acting as an adjective. The final clause is essential BY THE RULES OF how a linking verb performs in a sentence.
"The most important thing is that a great teacher is compassionate."
BTW, if you did use "was" that means an action started and ended before now, and that would mean the compassion of the teacher ENDED at some time in the past.
"The most important thing is that a great teacher was compassionate."
Our language is very complicated, you come to one realization after another as you work with the rules and discover your mistakes along the way. Mine is rushing to get this typed.
The reason I make a big issue out of the "linking verb" is that the noun clause as a predicate nominative, a noun, "identifies" or "explains" the subject, and the subject is "thing." "thing" is a pronoun that has as its antecedent (the word the pronoun refers to ) the noun clause on the other side of the linking verb.
You said, the most important thing is... Is thing "what you walked away with" or by the rules of grammar...is thing the noun on the other side of the linking verb. It is. The most important thing is not what you walk away with but the fact the teacher is compassionate. I'm done. Thanks for reading this.