Your experience isn't a one-off. Trying to go it yourself, without competent representation (appraiser, advocate, lawyer if necessary) most often ends up settling for ten cents on the dollar. While the process can drag on, it shouldn't be all that stressful or expensive. The independent appraiser you hire should have credentials, a state-issued Motor Vehicle Appraiser's or Adjuster's license from any state is preferable to a "certified" appraiser or one without any affiliations at all. When you receive the DV appraisal, instructions on the best way to proceed with your claim should be included. An important one is to alert the insurance adjuster that your appraiser would be more than happy to speak on your behalf. That appraiser should also provide customer service throughout the life of the claim. That means advising you how to respond to insurance company denials, low-ball offers and other excuses they throw at you in their attempts to make you go away. Expert witness testimony should also be part of the package, at an additional cost, of course. If your expert won't back up his evidence in court, at least via teleconference, defendant insurers will ask the judge to dismiss your case.
We have seen an auto diminished value case drag on for two years. The owner of the Tesla was content to take her time and wound up taking State Farm to court. I testified in that case. Our client was awarded $46,000.00 by the jury who also made State Farm compensate her for all legal, appraisal and expert witness costs.
No one wants to go through something like this but be assured that it can be you and not the insurance company that dictates the pace of the claim. Insurers are strongly encouraged to close claims and they certainly don't want to face litigation. They can't offer credible evidence to the contrary and they know it. Formulas, algorithms, online ad comparisons are all short-cuts that are easily discredited. Believe it or not, despite all of the scary things you read telling you that obtaining fair settlements is a rarity, you are in the driver's seat.