Short answer Defender is automatically suspended when 3rd party AV is installed. It is not easy to uninstall Defender and Microsoft made sure to allow you to use other antivirus without issues. You should leave everything as it is, leave Bitdefender installed and working, while Windows Defender will not intrude.
On endpoints and devices that are protected with a non-Microsoft antivirus or antimalware app, Windows Defender Antivirus will automatically disable itself. You can choose to enable an optional, limited protection feature, called limited periodic scanning.
When enabled, Windows 10 will use the Windows Defender scanning engine to periodically scan your PC for threats and remediate them. These periodic scans will utilize Automatic Maintenance—to ensure the system chooses optimal times based on minimal impact to the user, PC performance, and energy efficiency—or customers can schedule these scans. Limited Periodic Scanning is intended to offer an additional line of defense to your existing antivirus program’s real-time protection.
If you are also using Windows Defender Advanced Threat Protection, then Windows Defender AV will enter a passive mode.