It's always painful having to revert to a previous version when things go badly. Do you have a development environment and test suite in place? If you need help with the SQL or just a sounding board I've been developing databases and bespoke systems for a long time. I don't know Arc (and sorry but I'm not willing to spend time on it as I don't think it's going anywhere) but I do know Racket quite well.
Development environment, yes. Test suite, no. Test suite and CI would be ideal, but there are more important things that come first. Thanks for the offer. The SQL and schema side isn't complex or difficult though, it's changing the Arc code to use it. Which sounds easy, but there's duplicate data and unnecessary iteration at every turn. We're also slowly moving Arc code to Racket. The API is on github. We'd certainly welcome any pull requests. Right now, the big thing someone could do, would be to add API endpoints and SQL queries or views to return JSON data for all the information on a given webpage, or section of a page.
I'll have a look at that github repo over the weekend. Is that schema.sql file current? Are you using MySql/Mariadb/Postgres ... something else? edit: OK it's clear after looking at the source code.