Artificial Intelligence

What's your level of tech savviness?

It sounds like the letters are difficult enough to read by eye, and that provides certain challenges for transcription. However, some pre-processing of the image could sharpen that text and doesn't even require AI.

I can't really speak for the off-the-shelf solutions available online today because I don't really use them. They're virtually all privacy monsters. That doesn't mean they won't work or meet your needs, but be aware that if you use them then your great-grandparent's most intimate accounts would be put on some corpo computer and later used to sell something.

If you do feel comfortable with a little scripting, then I'd recommend OpenCV or similar to sharpen your images and some open source OCR library to do the transcription. I haven't done much OCR myself, but a quick search suggests maybe Tesseract would be easy to use.

If you're not into scripting yourself, but that sounds appealing, I'd be happy to take a crack at it. Those documents are important and worth preserving. Note that I only speak/read English though, so I might not be great at assessing the quality of the transcription.

If you don't feel comfortable sending a stranger your grandparents' letters then that's completely fair and understandable. But if that's the case then I definitely wouldn't be using an online tool for it