D&D, as a system, isn't really suited for classic swashbuckling fantasy. Swashbuckler doesn't work well next to knight in full plate. They are both fighters, but from very different eras. Classic d&d is mix of late medieval and early renaissance. Three musketeers are 17 ct, so baroque period. Zorro is even more modern, depending on media depiction, from late 18 to early-mid 19 century. Scaramouche takes place in late 18 century. Swashbuckler is a fighter, but fighter from age of gunpowder where firearms are norm, armor is less prevalent, and it moves to standardized military formations with muskets and artillery. In short, swashbuckler is best when used in dedicated swashbuckling genre and with dedicated system like 7th sea where you actually have mechanics to reward flashy shenanigans.
If we look at probably most famous Assassin from d&d novels, Artemis Entreri, we see he isn't really rogue/tief. In 2ed he was dual classed high level fighter/thief. In 3e he is mostly fighter (Fghter 12 / Rogue 4 / Ranger 1/ Assassin 1). Jarlaxle Baenre was mostly characterised as a rogeuish swashbuckling type. He is drow fighter 18 in 3e and fighter 17 in 2e.