Answer:
Keep the comma. In this sentence, so is acting as a coordinating conjunction that connects two independent clauses. An independent clause is a group of words that can stand on its own as a complete sentence. According to the web search results¹², when so is used as a coordinating conjunction, it should be preceded by a comma. A simple trick to test if so is a coordinating conjunction is to replace it with therefore and see if the meaning stays the same. For example:
Lily Truscott's band needed a new drummer, therefore I agreed to join.
This sentence has the same meaning as the original one, so we can conclude that so is a coordinating conjunction and needs a comma before it.