We are Locomotive and we're working on an open-source API-first ERP system, which will work really well with Retool.
In a nutshell, we will provide straightforward API endpoints covering a few typical activities (CRM, inventory, invoices, expenses etc.), which can be used as Retool resources. Using those, you can build a tailored ERP using the Retool app builder.
We have a demo up, which covers inventory management here.
The linked demo is done using an open source project called InvenTree. It worked for demo purposes, but we are not going forward with that codebase.
At the moment we're working on our own codebase, using Node, Nest.js, Postgres and Apollo. We'll provide a GraphQL API. For deployment we'll provide a Docker container.
Hope this helps. Let me know if you have any other questions!