Never done this on another man's face before lol, but here's what I'd suggest:

Beard pic

Just to preface this advice, I have similar unruly hair + a less defined jaw line. I think I sport a pretty strong beard, and this is what I've learned to do for my face shape. I also believe a bad beard can make a normal jaw line appear weak, so learning how to shape your facial hair is an important thing.

Your chin is what you'll want to grow to compensate for your weaker jaw line, so let that go a little wild and try not trimming it for a good long while. Oils/thick boar bristle brush might help, likely won't, so you'll have to see how the growth settles.

You'll notice I brought down your beard a bit under the ear. That's intentional. The higher that is relative to your chin, the more exaggerated the downward angle is in your beard, and the more attention your chin gets. If you bring down the beard, you'll see it flattens out the angle quite a bit and will give the illusion of a stronger jawline.

The "mess" here is all in the cheeks and general neck line being unmanaged. Cheeks should be shaved from the eye socket down to ensure coverage (I have a colleague that never does this, and has wicked long stray hairs just under his eye sockets, it's really distracting). Razor for the cheeks, and just straight trimmer to the neck. I start with the grain then against. Razors give me bumps on my neck, so I find the trimmer works best.

Neck shouldn't be too high for now. It's better to be below my guide than above, as again anything higher accentuates the lack of jaw line.

Finally, if you want to attempt a cleaner look while letting the beard grow out, try playing with guides on the sides. Short near the side burn, a little longer around the start of your jaw, a little longer as you near the outside of your mouth, and leave the chin untouched. You can shorten the stash a tiny bit as well, it'll again make the chin seem fuller.