Is it Legal to De Rate a 100 Panel to 20A?

[Coppersmith]

Just curious, what is the best DIRECT BURIAL cord for this application. It would be SE, not USE, right? For 120/240 VAC service ( 2 Hots and a Neutral) Can anyone recommend a specific brand or part number?
Now don't laugh, maybe I am not under standing the code, do I need an additional conductor besides the 2 Hots and Neutral to bond the metal Disconnect case at one end of the 387' foot run to the one on the other side?

For such a low amperage application, UF would probably be best.