Thanks for your feedback and suggestions.
Boss and Elite Battles are supposed to be the hardest, but it depends on your team composition.
Recycle is mostly useful for the endgame indeed, that's why it's the rarest event to show up.
Your units lose max HP depending on the damage they took during a battle so a valid strategy is taking intentional risks with Elite or Horde Battles to increase your RNG chances, since these give the most gold and items. That way, free repairs can be utilized to gain back the maximum health that was spent on obtaining more items.