Every chord comes from ONE major key — 1, 4, 5, 6-, 2-. No dominant 7ths, no borrowed flats.
the major scale (or its major pentatonic) — SCL·01
the relative-minor pentatonic (same 5 notes, darker landing) for the sweet-and-sad blend SCL·02
One key, one scale — the major scale can't play a wrong note here. Pentatonic is the safe core; add the 4 and 7 for color.