You should craft 2 new pickaxes and use them to repair your pickaxes on an anvil that way you get to keep the enchantments.
I'd wear the picks down a bit more first, as far as you can without worrying about breaking them.
Unless you have Mending on them there is a limit to how many times they can be repaired.
I think it's 6 anvil uses, and if you have used an anvil on them to get the enchantments you have that will cut down on how many times you have left.
Though with unlimited XP-levels making new ones isn't a bad option, both Efficiency and Unbreaking are relatively easy to get.
Since you don't have an awful lot of diamonds you're probably most worried about using them efficiently?
Unless you can get Unbreaking on a new pick the diamonds will last 4 times as long on the old picks.
Since it takes 4 diamonds to fully repair something from nearly broken and only 3 to make a pick, using a new pick for the repair is the better option, even more so for swords and hoes, armor should be repaired with diamonds but not 1 at a time, wear it down to 1/4 or so and use 3 diamonds at a time, or 4 if it's nearly broken because of the anvil limit.
Hey Hex, thanks for the reply I think it's sort of clicked for me now.
What I think I will end up doing actually is making a new pick, enchanting it with the highest efficiency/unbreaking combo i can directly, then combining book enchants to get exactly what I want before applying it to the pick. I think I'll leave the other 2 as backups available for repair if i'm ever out a pick.
I feel like the better enchants I'll get from combining the books will outweigh the anvil use I'll have to do.