yep, that ceiling fan is 100% your culprit - the constant airflow is cooling your layers too quickly on those longer prints making them brittle, try building a simple cardboard enclosure around the printer to block the airflow and you’ll see a huge diffrence.

Once PLA gets brittle like that, it’s usually because the polymer chains have broken down (hydrolysis) which no amout of drying can fix - the chemical damage is permanent.

Spot on about the Z-axis being the critical point of failure in most DIY CNCs - harmonic resonance at higher speeds will absolutley kill accuracy before anything else.