Does matter convert into any forms of energy or subatomic particles that aren't, or won't ultimately decay into, electromagnetic photons?
Of course, such process occurs in high gravitational fields (space-time curvature), where the matter spontaneously changes into radiation. The matter has it's own weak gravitational fields - so it's unstable, in principle. The most stable particles, like proton should decay in the range of 170 billions of years by Standard model and the Universe becomes free of observable matter.
As example can serve the radiation of the matter during accretion into black holes. By
AWT such process has occurred in the large scale during formation of matter at the very beginning of our Universe after inflation in so called quasars (super-massive black holes, resulting for compactification of vacuum). During this the excessive matter of quasars was separated by gravitational field into radiation and transfered to longer distance. After the gravity field has decreased, the part of matter has precipitated back again, thus forming the interstellar matter (the origin of galaxies). Some part of matter rest dissipated in the vacuum as so called dark matter. We can even observe such process at very distant and old quasars, which are very radiative. The more closed quasars have radiated the excessive energy and they have been transformed into cool black holes in the center of galaxies, so they cannot be observed so easily.
It's obvious, such process is solely reversible: in strong electromagnetic fields, the light spontaneously decomposes into matter under formation of particle-antiparticle pairs and the formation of matter (so called "energy materialization"). The resulting particles are having its own electromagnetic field often, so at the presence of excessive energy (i.e. the photons of high energy gamma or cosmic ray radiation), such process is "auto-catalytic" and the particle formation process appears in cascade, until the energy is depleted. It's similar to the vortex formation process in fluids. It should be pointed out, the very same things are valid for gluons and W/Z bosons, too.
Unfortunately, the gravity is longer distance interaction than electromagnetic interaction and it has no charge sign, which enables the action compensation - so such equilibrium is shifted towards photon formation and the matter is unstable with the contact of vacuum, so it's evident, it doesn't appeared in thermodynamical equilibrium. The matter is rather result of fast adiabatic cooling of Aether, simmilar to the preparation of unstable compounds by chemical reactions.