A Spellsword is a mix of fighter and mage. Since they had learned basics spell casting and combat skills they are not particularly good at casting and fighting. They like to be offensive slinging spells while the close the distance on there target. What makes them dangerous is there versitility.

Sub classes -

Oathblade - focused on defending ally's. Gains honor from there oath for keeping an ally alive to fight another day.

Reaver - focused on offense, the more enemy's surrounding them the stronger they get. Don't get cocky though.

Subclass upgrades (lv 10 and lv 15)

Champion(lv 10) - your battles have sharpened your senses and increased your power. You become more aware of your surroundings, gain a skill based on how you play, and +2 to a stat based on how you play.

Spellslinger(lv 10) - you have become more proficient in casting spells, increases your wisdom by 3 and decreases chance of spells missing or failing.

Headstrong(lv 15) - damage is nothing to you, rushing into combat has greatly increased your constitution by 4 and your strength by 2.

The dm will decide when your character gets these buffs and will choose which ones you get.

I think there's some room for this, but how does it differentiate itself from the current half-caster classes (Like Ranger, Paladin, or Artificer?)

It seems like there's not a lot to make this a distinct class... I feel like you could maybe do this same basic concept just by playing as an Eldritch Knight Fighter.