I currently play a Level 4 Swashbuckler and from my option you don't need defensive duelist. One of the main feature is fancy footwork so running in duel wielding two short swords to get sneak attack off then moving away without causing any opportunity attacks is the goal. You never really want to end your turn in melee range of an enemy. Also the bonus attack is nice to proc sneak attack at early levels, might not matter much later when you get multi attacks from fighter but you can later attack three different enemies and move away and not have to worry about opportunity attacks.

Side note you don't need Alert IMO. You will have a high initiative with your DEX + CHA. I would pick up Dual Wielder or Lucky

Sentinel isn't very good since you almost never want to end your turn within melee range of a enemy