After many years playing guitar, I'm unable to craft worthwhile (or play, for that matter) shredding, thrashy solos. I'm stuck at learning pentatonic scales without really knowing how to apply them. Let's say my influences range from old Obituary, Death and Autopsy to Possessed and some other usual suspects. Lots of borderline deathrash as well.

So what do you recommend? Certain solos, techniques, scales, exercises...?

What kind of solos do you want to play? You say you're stuck with pentatonic scales, but the riffs you're coming up with aren't all pentatonic, I'm assuming?

Do you know what mode your riffs are in? You don't need to know the official name, but do you know which notes are part of it? Most style of music have guitar solos, that are just more complex melodies, in the same mode. If you want the solo to sound really different, make its root a different note, on your scale. That can give you a whole new sound. But if you choose to have the same root, just make it a more over the top version of the main riff. Try out different licks, by using the same scale, but putting licks in different places.

The melody can go crazy, up, or down, the fretboard. Or, it can be a simple melody. You can add ligatures(hammer on, pull off), bends, vibrato, whammy bar. Just make sure these accents are in the right place, so the whole thing sounds good. Put delay on it, or wah, or some other effect, see how it sounds.

If you want to use chromatic notes, you can. Just ask yourself whether it sounds good. If you want to shred, teach yourself the basics first. Shredder guitarists have skills you don't have. But the way to develop those skills, and to make shredding sound good, is to teach yourself how to make solos that sound good.

You said you are influenced by these bands, so I recommend learning their solos, by tab or by ear. Chuck from Death has certain phrases and scales he uses a lot that if you learn his solos you will have ingrained in your playing. Specifically, he take a scale (lets say the minor scale, or Phrygian scale) and play it with three notes per string, but rather than just go straight up or down the scale, he will play a pattern (3 2 1 2 1) for example.

Scales: minor, phrygian, dominant phrygian, harmonic minor, dorian #4, hungarian minor. (They all sound very dark and have lots of half steps, all but the last two are extremely common in metal)
Exercises: Learn songs