Hello Sk Azmaeen,
Welcome to Microsoft Community.
Please allow me to answer your questions briefly and clearly.
1. Can I use "Dell D6000" dock station with "Microsoft surface pro 9" via USB C ?
We do not recommend that users use third-party docking stations, as this may lead to incompatibility issues. We only recommend Surface users to use Surface-specific docking stations:
Buy Microsoft Surface Dock 2 – Microsoft Surface
Buy Surface Thunderbolt™ 4 Dock (Ports, Compatibility, Price) - Microsoft Store
Buy Microsoft Surface USB-C Travel Hub (Ports, Compatibility, Price) - Microsoft Store
2. Why is there a slow charge warning on the Surface, but when I connected the power adapter, the warning disappeared.
This is mostly because you are not connected to the Surafce Pro 9's dedicated power adapter via the Connect port link, but your DELL dock is connected to the monitor, and the monitor is connected to the power supply. This causes your Surface to detect power via the USB port and lets the Surface Pro 9 determine that you may be charging via the USB port at this point, but because the power is already consumed by the display, this prompt appears.
When you Connect the Surface Pro 9 charger, the problem doesn't exist, because Connect port charging is prioritized:
USB-C and Fast Charging for Surface - Microsoft Support
Therefore, Copilot's answer is actually right.
3. If I use it too much or like more than 10 hours a day, is it going to impact the battery health?
If you would like to extend your battery life as much as possible, please allow me to share with you some information from the following links:
Maximize your Surface battery life - Microsoft Support
Caring for your Surface battery - Microsoft Support
Best Regards,
Mitchell | Microsoft Community Support Specialist