Increasing xp dropped by mobs?
Solution 1:
Loot tables can't contain XP.
The best you can do is to make an advancement with the minecraft:player_killed_entity trigger (archive), the conditions you need and experience points as a reward (archive). The advantage or disadvantage, depending on what you want, is that the XP won't be dropped as an item, but given directly.
If you want to drop the XP, you have to do what you already knew: Detect the kill and summon XP orbs yourself.