This is my personal opinion which seems to work with players
I’ve always ruled the spell effect as being able to levitate a creature or object that weights 500 pounds or less and that’s it as far the spell’s weight limitation goes.
Whether a PC can carry someone or something else depends on their ability to hold onto the weight they want to carry. Just because you’re currently under the effects of Levitate doesn’t mean you can personally carry more. That extra load requires you to hold it so the character’s strength score is going to come into play.
Alternatively, you can add the (commonsense) restriction that if the total load being lifted exceeds 500 pounds the spell ends, with whatever effect the DM things is relevant happening e.g. fall to the ground, drift down, etc.
So, a PC under the effect of Levitate, to my mind, can’t say “I want to carry up that 300 pound boulder and drop it.” unless they can personal carry that weight normally.
I’ve found that most players I’ve played with agree with this approach as practical and still useful in game.
Carrying an unwilling target
For this I would rule that a standard grapple check is required. There is nothing mechanically different about grabbing someone while under the effects of Levitate compared to grabbing them normally. The effects of the grapple are still the same as are their options fro breaking free so I see no reason to further complicate it.