Comma before 'since'

[Dimi1]

Hello fellows, I haven't quite understood 1 editor's comment on a review and I would your feedback if possible. I'll give you the sentence and explain to you my thinking.

The sentence is "this isn’t important, since we are speaking mainly about an erotic romance novel with the mystery coming next".
Editor says: "Unnecessary comma before subordinating conjunction".

Given that "since" is subordinating conjuction and follows a negative independent sentence ("this isn’t important") I considered that for reasons of clarity a comma should be put. If there wasn't a comma it can be implied that there was no relationship between the unimportant thing and the priority given on an erotic romance novel. Without the comma one could imply that it is important, but that the priority on erotic romance was not the critical factor contributing to the unimportance of other things. I thought we had a similar rationale with when to use a comma before 'because', that's why I followed the relevant rule provided in grammarly (https://www.grammarly.com/blog/comma-before-because/). Do you find it wrong? Any enlightenment would be welcome.

[Juliet+1]

There are a lot of situations where a comma is not required but is allowed, and I think this is one of them. I like the little pause after "important." It makes the rest of the sentence easier to read.

[Diana Lowery]

The "rule" for this situation is that when the subordinating clause comes at the beginning of the sentence, you put a comma after the clause; however, when the subordinating clause comes at the end of the sentence, you do not put a comma before the subordinating conjunction.
As with all English grammar rules, they are sometimes subjective.