I don't know whether you would like to disclose the whole source and what you would do if the product takes off, so didn't explicitly ask you to open source. But yeah, show users what was sent would be helpful, better if there's some way users can verify.

I like your interface aesthetics.

Instead of asking users to download data and reupload, maybe animations using actual CSVs and query artifacts (graphs) are more illustrative for people in a hurry or people who cannot come up with a query question atm?

And the privacy bit would be more convincing if people can see that part of code.

I've used {GNU |X}Emacs intensively for over 20 years and never thought about doing a thorough study of many of the things it provides like you do in Emacs Solo. Your work is inspiring to me in showing me what dedication and thoughtfulness can achieve, even when it seemed at the beginning as trivial as one tending their own homestead. Many, many kudos.