I'm working for company PROFICIO which has 900 clients, 6 000 projects and 24 000 tasks in Toggl. We tried migration with API approach, but it failed. So we switched to native Toggl export and native Clockify import (we just need migrate clients, projects and tasks). This script will convert JSON to CSV format. I used PHP and  SQLite (allowing simple edit via Adminer) for migration.
- Clone repository
- Download Toggl Workspace Export (json) to datafolder
- Follow steps 1-4
- Use Clockify (workspace import)