Gieco Claim Diminished Value Negotiations

Has anyone had success negotiating with Gieco? My Mercedes was plowed into by one of their customers and I have spent the last 6 months with their claim process (12 weeks plus my car was in the shop 2018 GLC300 SUV). The last piece was the diminished value claim and Geico offered me $600. I said that was ridiculous and that anyone who had the choice of buying my car or one for that wasn't in an accident would not choose mine for $600 less. I was told to do my own research and submit documentation to prove my position. I paid an appraiser who does this for a living $400 for the appraisal and her diminished value came in at almost $6000. After one month of no response after uploading the official documents I called today and the Geico adjuster said she finally looked at it and said all the comps were inflated and her offer of $600 stands. After speaking with her supervisor and having a ridiculously long sparring session he said he would go up a little but I need more documentation. I am told this is the game they play and when I went back to the adjustor I hired she said to 1. Go to court with or without an attorney, 2. File a claim with the AZ Insurance Board 3. Go to the Mercedes dealership and see what they would give me for a trade-in and use that document as further proof in court. 4. Carmax online also for a quote. Apparently Geico offers 10% of the value of the damage (mine was $6000) which makes no sense as far as value goes and they are FIRM they don't use any of the formulas you will find online. Has anyone had success with Gieco in this category? Feel like suing for all of my lost time spending on this as well as the cost of the appraisal which also should have been their expense. Appreciate any feedback!