I like Option 3 Since it will just need to add an additional invoked skill of 100% boss dmg to existing skills.
Since Ice Arrow 3 and Tempest Shock 3 are readily available right-off-the-bat after class change and requires just 1 SP to get the 100% boss damage, just add the 100% boss damage buff in one of the the readily available skills for all melee and every ranged physical after their respective 3rd class change also.
Example
Sniper: Dextrous Shot 3
Sharpshooter: Dextrous Shot 3 or Breach Fire 3
Wind Walker: Dextrous Jab 3
Assassin: Spiral Jab 3 ( Invoked skill duration may need some change if boss damage is added)
Templar: Torrent Slash 2 or Deviated Crash 3
Crusader: Grave Wounds 2 or Beastial Impact 3
Soul Blader: Tyrant Strike 2 ( Invoked skill duration may need some change if boss damage is added)
Champion: Wind Cleave 2 or Deviated Crash 3
Vanquisher: Comet Strike 2 or Shockwave 2
Myrmidon: Tyrant Strike 2 ( Invoked skill duration may need some change if boss damage is added)
Let us not forget the healers in case they want some buff too.
Saint: Devine Arrow 3
Prophet: Destructive Incantation 3
Since the skills are the very first thing available after class change and will just add an additional invoked skill of 100% boss dmg there is no need to change the skill build of players.