This really feels almost like either a control issue, in which case use a lot of FromSofts controller set up combined with the radial, or something that got cut for time. Being a swashbuckler with a sword and pistol, both being straight in the ranger tree, and not being able to parry is just... Not very swashbuckle-y? It makes me not want to try and play that class. As other people have said, parrying with a one handed weapon means either having no offhand, in which case just use a two hander that'll deal more damage and stun, or a shield.
Most frustrating for me is not being able to parry as a spell blade. There's neat combos of frost sword/frost grimoire and fire sword/fire grimoire that I would love to play, and each of those swords have a parry upgrade that I want to use to spread more of the condition around. But I just can't... So I wind up back on wand/grimoire and feeling limited or unfulfilled. One of my biggest gripes with the game (which says something about the rest of the game being pretty dang good).