I can confirm FP5 support USB-C DP AltMode 
- default setting is mirror phone screen
- can be enabled DesktopMode in Settings/Display
- lcd is suspended and resumed together with phone screen
Tried with portable lcd âLenovo ThinkVision M14â (Non-Touch version),
direct connect to FP5 via USB-C <> USB-C is enough to power lcd,
without external powerâŚ