If you live in the Apple ecosystem, Agenda(1) could be just the right tool for you. I also very much like their pricing schema(2), more apps should do it like this!
You can export whole projects as one-file markdown (or HTML, RTF), worst case being having to go through every project, exporting and splitting the file up (by a script), or turn the agenda folder somewhere in ~/Library (contains bunch of json) into something usable via script, but I agree, it lacks a reasonable solution for getting it all out at once.
1. https://www.agenda.com/
2. https://drewmccormack.medium.com/a-cash-cow-is-on-the-agenda...