if you have mending you use XP to repair it
Repairing of pickaxes

it's very easy to get mending, find yourself a village or cure a zombie villager. Get a lectern, and give an unemployed villager the lectern. Keep breaking the lectern and putting it back until the villager is offering mending

I basically use stone the most unless ores which is then iron and obsidian and ancient dribes diamond

I would fuse other enchanted picks together when they are almost broken on an anvil to add up the enchantments

I use stone for everything other than ores which I use iron and then diamonds for obsidian, diamonds, and ancient debris. also I never used a diamond pick other than to mine obsidian and ancient debris to be honest and only twice until I got netherite which is used same as a diamond pick

Yes, "easy" very easy. 10 thousand years later
well depends I'll admit. But in the end it's always worth it, you have an unlimited supply of mending and whatever other enchants you need. I have a sharp 5 villager, eff 5 villager, unbreaking 3 villager, feather falling 4 villager, prot 4 villager, looting 3 villager, sweeping 3 village, depth strider 3 village, and a fortune 3 village. I always have the books I need because I spent some time breaking and placing lecterns
Is it better to repair diamond pickaxes using diamond pickaxes or diamonds if you do not have mending?