No doubt the junior dev would benefit a lot more from only getting up to 60-70% code coverage. The extra time could then be used to learn basic Db performance,

No doubt the junior dev would benefit a lot more from only getting up to 60-70% code coverage. The extra time could then be used to learn basic Db performance, which is hugely important when tables have millions or billions of rows. Stuff like indexes and isolation levels should be taught to all junior devs, it's just too risky not to.