Black holes for example, while having themselves lower entropy than environment,
are the largest catalysts of entropy increase because ~40% of matter
falling into them is converted into light.

I may be a little bit pedantic but do you mean light or gravitational
waves. If a mass falls into a static BH 50% of the mass is converted
into gravitational waves and 50% of it increases the mass of the BH.

A spinning BH is extremely interesting. Here up to 80% of rest mass
energy may go into gas jets. This is the main energy source of abnormal
galaxies.