Is a drop of prostatic fluid (madhy) that does not extend beyond the tip of the penis considered among the minor impurities that are excused, and how can one get rid of the obsessive doubts about the transmission of ritual impurity (najāsa) and purify the body, clothes, and home?

You must repent to Allah for the actions you mentioned. Madhi (pre-seminal fluid) is impure. You stated that you used to pour water on theja (impure) spot, and this is sufficient for purification. Water that separates after removing impurity is considered pure if its color, smell, or taste has not changed due to the impurity. As for semen, it is pure.

You should not deem something impure unless you have absolute certainty. If you doubt whether impurity has transferred, the original state is that it has not transferred. Know that a dry impure object does not transmit impurity to another dry object it touches. You must cast aside obsessive doubts and not give them any attention.