What is the most effective way to restore durability with Mending in Bedrock Edition?

Generally, the best way to repair mending items is via an XP farm while not wearing other items with Mending on them. For now, the following behavior is "working as intended" on Bedrock:

• Mending only repairs equipped items
• You can only have a few select items equipped in your off-hand
• If you have multiple items equipped with mending (e.g. armor + a main hand), the game randomly selects one to mend. If it's fully repaired, the game doesn't select another item. Instead, it adds the XP to your XP bar.

In Minecraft, you gain experience by:

• killing mobs
• mining ore
• smelting
• breeding animals
• fishing
• trading
• disenchanting items in a grindstone
• drinking bottled experience

Of these methods, the two that tend to be the easiest to automate are killing mobs (via a mob farm built at a mob spawner) and pulling cooked kelp out of an auto farm/smelter. Set one of those farms up, make sure you equip 1 mending item at a time, and collect XP from the farm.

I use a mob farm to repair netherite gear with Mending. In my experience, a mostly damaged item takes ~10 minutes at the farm to repair.

I also have a trading hall and iron farm in my world. To gain emeralds, I usually sell a bunch of iron each session to a group of armor/weapon/toolsmiths I've installed in the trading hall. That usually nets me enough experience to top off lightly-damaged items. However, for heavily-damaged items, the mob farm is much more efficient.