A few observations on the facts as reported by the OP:
The title refers to a “barney” with ATC - as has been previously highlighted, it takes two to tango, and it achieves nothing by being abrasive (pilots or ATC) but everything by knowing procedures and adhering to standard communications.
Turned left, apparently without informing ATC - moral, request or inform ATC (depending on the perceived urgency) before the event - e.g. “request turn left maximum 20 degrees for terrain clearance” - that would immediately keep ATC informed of the maximum required track deviation and give the explanation in the same transmission. Clearly, if that were still refused, it would then justify an urgency call.
ATC insisted turn back on track - would have been useful if ATC had specified a reason or passed relevant traffic information, BUT perhaps the reason was pressing at that particular moment, e.g. co-ordination of following departing traffic or other potential conflicting traffic on a different frequency.
“I wasn't happy with the clearance I would achieve” - perhaps this was misunderstood - did you specify terrain clearance; would a mention of rate of climb or anything including unable have been more emphatic? Remember that ATC was not operating in her first language; you, presumably were and also deviated from appropriate communications by making a remark about “flying into a mountain to keep her happy.” How would that sort of response sit with you if communications had to be in Spanish by both parties - even if understood, how does this contribute to a safe operation?
“She was pissed off...” - by your remark, quite possibly, BUT how do you know that? What might have been evident in her tone was concern about some urgent unexpected co-ordination that she needed to effect.
“Handed me off to a radar service early to get rid of me...” Should you not be grateful? It sounds as if the aerodrome controller (who would not necessarily be aware of all potential conflicting traffic on your new track) effected some swift co-ordination of your revised requirements and transferred you to the more appropriate frequency, i.e. for a potential radar service, in a very timely fashion.
“There was only one other aircraft in the area a guy practicing ILS approaches that was nowhere near me”. Situational awareness is one thing, but you cannot know that - or know what else might be nearby in a few miles’ time.
“I suppose I should have done my calcs prior to take off but didn't”. No comment.
2 s