This tool is very powerful! Just what I needed, thanks again. Turns out you need pandoc 3+ and linux mint repo has 2.9, so after upgrading that rga started working, but it still throws this error from time to time:

parseSpine
Error: copying adapter output to stdout

Caused by:
    0: subprocess: Command { std: "pandoc" "--from=epub" "--to=plain" "--wrap=none" "--markdown-headings=atx", kill_on_drop: false }
    1: ExitStatus(unix_wait_status(16384))

I will have to keep looking into it, I'm not sure if this error stops the search in it's tracks.