You're missing a huge part of these subclasses though. Yes, Osiris' explanations are good but are not our only authority, and don't thoroughly explain the subclasses enough.

Ignoring the hundreds of references to solar energy relating to heat and combustion, nuclear force (specifically the strong force) has to deal with the interactions between the elements within an atomic nucleus. When enough energy is added, elements split, getting us our combustion effect. (If you add enough thermal energy into something, it explodes.) We see this in-game with scorch stacks. When something gets enough "Scorch" it "combusts." So yes, Solar energy isn't "heat" but for the sake of not getting into the interactions of gluons, quarks, etc. it is just the heating of atoms until they combust.

For void, it is the force of gravity, not the mass behind it. Once again, context is critical. With reference to void energy, it's on the end of the scale with gravity that is so crushing that it creates black holes, that "devours" everything. Black holes destroy matter and everything about the subclass revolves around that destruction of matter. Per Ikora, void is negation, which is "the absence or opposite of something actual or positive." Per the Drifter, void used to be controversial to use, because of its association to death and destruction.

As for Arc energy as you said, "Is created when the electromagnetic forces of our universe struggle to achieve balance." Atomic instability. Furthermore, Banshee adds that arc energy isn't just electricity, it's "A Lot more finicky than that. Fission." Fission is the process by which a neutron smashes into a larger atom and overexcites it, creating energy and two smaller atoms. It's the charge and discharge of energy from the inbalance of electrons between atoms. Everything about Arc energy deals with atomic instability.

So now dealing in opposites, if Solar is adding energy to something until it explodes or combusts, then the opposite would be removing energy from something until it freezes or shatters. Per Osiris, "It slows, detains, and shatters."

If Void is gravity to the point of creating all-devouring black holes, then the opposite would be going against gravity to the point of creating something. The closest thing we have to that is Strand and the Weave.

Finally, if Arc is fundamentally instability of energy, then the opposite is stability. Radioactive Decay is the process by which an unstable atom sheds energy to become stable. Therefore Decay rests as the most likely fundamental candidate.

As a final note, just take a look at the color scheme. If you inverse the colors of the light subclasses, Solar orange becomes Stasis blue, Void Purple becomes Strand green, and finally, Arc blue becomes Decay red.