Metaphysician |
A mechanic is a mechanic, you can't be a Mechanic 3/Mechanic 5 anymore than you can be a Soldier 4/Soldier 2. Multiclassing requires you to take different classes, not the same one.
( . . . I do kind of imagine that, if people are still playing Starfinder 20 years from now, "Mechanic should have been two classes" will be one of the ongoing angry controversies. . . )