I actually wrote a script like that not too long ago you can probably use.
The way this works is you use the Grafana API (docs are here) – it has a search API which lets you identify all of the dashboards that you’ve got. You then use a different dashboard endpoint to automatically grab all of the JSON that defines the dashboard. This is the same as going into the UI, and then inspecting the dashboard and saving its JSON configuration, we’re just doing it all at once for all the dashboards.
This script will not work out of the box for you because it runs against a public Grafana. For yours, you will need to check the authorization section the API docs and in the bash script make sure that curl is submitting the right bearer authentication token, but the basic approach here will work for bulk-export all dashboards → JSON from any Grafana instance, when you have the auth correctly configured.