Why can't this be addressed like Blizzard has with Diablo Immortal?
Compute the average level/XP total and increase the XP rate for jobs, duels, and fort fights via a buff for those players that are woefully behind the world's average level. This buff would of course be throttled as you apporach and then disappear once the player gets within a range of the world level/XP total average. Also this buff would supersede any other buffs outside of gear enhancement.
So for example if the average level is 178 and you just start the world, your XP buff would be 1700% until level 17 and then drop by 100% every 17 levels until you hit 170 after which he buff is removed from that player. Any buffs like "Experience boost (usable)" or "Potion of Some Wisdom" can be consumed but will have no effect while this buff was active.