Would one be sinful for refraining from attending a Tajwid course, preferring to study Tafsir instead, given their proficiency in Tajwid rules, and out of shyness from the Sheikh due to his harshness, and does the Hadith apply to them: "One came forward, the second felt shy, and the third turned away, so Allah Most High turned away from him"?

There is no sin upon you for missing the lessons of this shaykh. The hadith of Abu Waqid al-Laythi does not consider everyone who misses gatherings of good as sinful. His statement, "And one turned away, so Allah turned away from him," could mean that the one who turned away was a hypocrite or someone with a sickness in their heart. It could also be due to an impediment that caused their deprivation of good. The harshness of the shaykh should not deter one from seeking knowledge.