Is it sufficient for ghusl from janabah to make the intention and say "Bismillah," then go under the shower without rubbing until it is highly probable that water has reached all parts of the body, in order to reduce the time spent in ghusl?

The matter of ghusl (ritual bath) is easy, and the method mentioned by the questioner is sufficient. If it is highly probable that the water has covered the entire body, then the state of ritual impurity has been removed. However, if it is possible to perform the complete, sunnah (prophetic tradition) ghusl without falling into obsessive doubts, that is more preferable.

This is a hardship caused by obsessive whispers (waswas), so the questioner should ignore them and pay no attention to them.