I would want to adjust multiclassing as well. Split the classes into caster/non-caster groups. If you multiclassing, you can have only one of each group type in your multiclassed character. This would strongly cut down on the OP combinations.
I would want to fix -5/+10 by adding the text "Once per round on your turn" to the ability.
Another thought is to make it -5/+"2xProficiency". That means that it starts as -5/+4 and at high levels is -5/+12.
Possibly both of these things.
The vast majority of people I've seen try it just end up reducing their damage. It's definatly caused more harm than good.It's not broken, just far more situational than it used to be. You use it when the ammunition on your one attack is limited in some way - it's a special arrow that you don't want to miss with, or it's a ranged attack spell and you don't want to miss and waste the spell slot.
That being said, I've yet to take it on any of my casters because I can usually arrange for Advantage via other means. But I might.
I can't comment on that, as I've never seen anyone actually use it.The vast majority of people I've seen try it just end up reducing their damage. It's definatly caused more harm than good.
And there aren't many attack spells, with scorching being a multi-attack.
Maybe if it was a level 1 spell, bonus action, you gain advantage on your attacks this turn.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.