Use a set-in-stone rubric. Everything you've listed there is entirely subjective. It's very possible and feasible to create a rubric that scores your kids FOR you, essentially.

What I use:

Four main categories are graded. Tone quality, notes, rhythms, musicality. Tone is worth 40%, each of the others is worth 20%. Grade each 1-10.

A kid with great tone (9) with mostly right notes (8) and rhythms (8), with attention to dynamics (9) earns an 86. Done.