Hi [Customer Name],
You are correct cloud flares page rule do not suppport user-agent based redirection. However you can achive this functionality with cloudflare workers.
You can however deploy a Worker and bind it to the route you want to protect. This allows you full control over how requests are handled.
I have attached the documentation for cloudflare workers and headers below.
Workers Documentation
Headers Documentation
Let me know if you'd like help deploying this Worker or customizing the logic.
Thank you
Cloud Flare Support
Thought Process used
Developer documents, community forums