@shadowcat5 Thank you - You're very lucky you're able to do your hair at home, it's definitely a safer bet and you know what works for you

I think in this case the toner won't cut it (they tried twice) because I want to get rid of the black, but now I have black with yellow and mint strands all over.
@OnlyHereForTheLolz Thank you - Yep, I have the mint / yellow streaks at the back. I actually took pictures of what my hair used to look like before and I asked them to pull up my references and they said they didn't find them. 100% my do before never classified as a "balayage" (or they had been severely undercharging me for years which I doubt).
I have been going to this salon for 6 years (except for when the pandemic happened) and I've always had amazing results. I once had a mishap 6 years ago with them and the then hairdresser was so nice despite the situation. We even laughed about it and it got sorted the following week. This one just had a poor attitude throughout the process. This is not how you behave with a customer.
I think what would turn me off from this salon is the hairdresser's attitude (reluctance to listen to what I wanted, did some random work without even assessing whether it would blend together, blamed me for the mishap and then did not offer a kind word / apologized / offered to rectify and completely ignored me on my way out). You're right though, I will ask for another stylist (a more senior one) and if they suggest she does works on it again, it's a big no-no.
I understand hair grows back. In my case my hair was super damaged before and it took me ages to grow it back for it to be in a pristine condition like this. She damaged my "virgin" hair (i.e. not tinted at all anymore) and now I'm going to have to rectify it so God knows the damage it will entail. It was a blank canvas and she messed it all up - that's what pains me the most. Hair grows back, but it's still painful when you've put efforts into restoring your natural state of hair and someone ruins it unnecessarily.
Not to mention - I'm usually pretty invisible in crowds, but I swear today people were starring at me right left and centre. It was uncomfortable. I look like a Halloween costume lol.