I’m genuinely curious what you were trying to convey by completing your, totally valid, disagreement with “so whatevs”? I believe this is the part that’s perceived as rude because the expansion of that, “whatever”, is often further expanded as the sarcastic form of “whatever you say”.
I was going for something between YMMW and "whatever you say." The slight tilt toward the latter was received poorly ¯\_(ツ)_/¯ It's just how I talk. Maybe it's generational.
To be fair, I didn't read it as condescending. This
I understood it as a generic statement ender, like "idk" or "ymmv" or "i guess", i.e. something that you put at the end of a sentence when you don't know what more to say. Maybe it actually is generational?