Yes, you can use 2 4G SIM cards at the same time in Nokia 8.1.
Yes, Nokia 8.1 is the metal body phone.
Yes, Nokia 8.1 does have the 3.5 mm headphone jack. It also has Bluetooth A2DP support.
Yes, Nokia 8.1 does have the HDR10 support and you can watch 4K videos on Nokia 8.1. You can record 4K videos also.