Is the release of a small drop of pre-seminal fluid (madhy) after a lustful twitching of the penis considered [a violation of ritual purity], even if the erection itself was not due to lust? And is it obligatory to be strict and twitch the penis?

There is no legal requirement to inspect or pull on the penis to ensure nothing is inside it. Rather, this is an act of extremism and innovation (bid'ah), and closer to obsessive-compulsive disorder (waswasah), as mentioned by Ibn Taymiyyah. The difficulty you are experiencing is a result of yielding to these obsessions, so fear Allah and completely turn away from them. As for what came out of you, you are more aware than anyone else of whether or not you felt pleasure. The important thing is that if it was madhy (pre-seminal fluid), it invalidates ablution (wudu) and requires istinja' (washing the private parts). If it was maniy (seminal fluid), it necessitates ghusl (ritual bath) and is not considered impure according to the most authoritative opinion.