I do think the difference between an Oathbreaker paladin as a sort of anti-paladin class with powers of its own and just any old lapsed paladin who has broken their oath is a potentially interesting one. This is partially engaged with through game mechanics in BG3, insofar as merely breaking their oath just leads to an inability to channel divinity (though admittedly they can still cast other divine spells), whereas getting Oathbreaker powers requires explicit commitment to a new course to the Oathbreaker knight. The details of this are left hazy, which is probably necessary given the variety of motivations an Oathbreaker might have, but for me it should at least mean explicitly rejecting the previous oath and now seeing elements of it as something that it is righteous to actively fight against, with a degree of zeal that could attract divine favour.
I do get some of this feeling from the spiel of the Oathbreaker knight as well as the game mechanics, though admittedly I think it could come across more strongly.
