There are a couple of things you can do to try to decrease render time. For example, you could try decreasing the number of volumetrics by using the glare node rather than volumetrics to create glares around lights, by faking clouds with textures or images, by replacing 3D explosions with 2D explosion special effects, by creating hazes with planes that have a transparent cloud texture on them, or by simply eliminating some or all of the volumetrics altogether. It all depends on what you were using the volumetrics for. Of course, it won't look as good as with volumetrics, but that's the price of shorter render times.
Another thing to try is using the denoiser node and reducing the number of samples you use (and the number of light bounces). For best results, check "Denoising Data" under Properties -> View Layer Properties -> Passes -> Data. However, you're still beauty for time.
Whatever you try, there's ultimately no solution for long render times except progressing Computer Science research and render farms (hundreds or thousands of high-end computers working together to render animations). Actually, 15 minutes is pretty good, even for an animation. It takes Pixar about twenty-four hours to render each frame for their animated films. The only way they can render the entire animated movie in a reasonable amount of time is by using huge render farms of thousands of computers working together.
The good news is that there are several online render farms that you can use. The bad news if that they cost money. However, they're still a viable option (better than half a year of rendering for you). You can read more about some of those options here:
Hope that kind of helps, and sorry I can't give you a better answer.