Will we ever find a way to unlock 100% of the energy available in a mass?

You can get decent conversion efficiency (in theory up to 50%, in practice more like 10%) using black holes.

Method 1: Attach a volunteer to a very strong spring and have him bungee-jump into a black hole. In principle, at the event horizon the stretched spring contains energy equal to 50% of the volunteer's rest mass. The volunteer lets go of the spring and disappears into the black hole. The spring can now be used to drive a motor or perform other useful services.

Method 2: Send the volunteer down a waterslide with a black hole at the bottom. The speed of the volunteer's descent will heat the water, again with a theoretical limit of 50% of the volunteer's mass times c squared. Afterwards the hot water can be used to heat homes in the winter.

Obviously there are some practical details to be worked out in these methods, but they give you an idea of why astronomers think quasars with the brightness of a thousand billion suns are really large black holes. Stuff is falling into the black hole and releasing some of its energy through friction (like Method 2) causing the colossal blaze of light we see.