Have you tried the --initial_prompt CLI arg? For my use, I put a bunch of industry jargon and names that are commonly misspelled in there and that fixes 1/3 to 1/2 of the errors.

I was initially going to use Azure Cognitive Services and train it on a small amount of test data, after Whisper released for free I use Whisper + openai GPT-3 trained to fix the transcription errors by 1) taking a sample of transcripts by Whisper 2) fixing the errors and 3) fine-tuning GPT-3 by using the unfixed transcriptions as the prompt and the corrected transcripts as the result text.

Whisper with the --initial_prompt containing industry jargon plus training GPT-3 to fix the transcription errors should be nearly as accurate as using a custom-trained model in Azure Cognitive Services but at 5-10% of the cost. Biggest downside is the amount of labor to set that up, and the snail's pace of Whisper transcriptions.