Well, you could try some of the established services:
- https://chat.mistral.ai/ (https://mistral.ai/news/pixtral-large/)
- https://claude.ai/
- https://chatgpt.com/ (or https://copilot.microsoft.com/)
They all want you to sign up but offer some free service. Try one of the letters you already have a transcription for, and see how well they fare. I don't have the slightest clue if they can read Sütterlinschrift or whatever your great-grandfather used to write in. You'd take a picture or scan, upload the file to the chatbot and add an instruction to transcribe the handwriting. And keep us posted if you do.