I have other mending items, but they are at full durability. Does that matter?
No, it doesn't. Even at full durability any equipped piece of mending gear may be selected randomly for repair. Since it can't be repaired you just get the exp instead. I make sure I have any mending piece I want repaired in my offhand and no gear on to focus repairing.
Lately, I mine with Silk Touch. Once I get 2 or more racks of coal/redstone/diamonds, I go to a safe space, get naked, and lay out the blocks and mine them again with Fortune III (with the tool to be repaired in my off-hand). Getting naked is an important part of the process. Same with the AFK fishing machine; it works best when naked.