As you can see, the speech spectrum is fairly wide, ranging all the way from around 250 Hz up to 4-5 KHz. So amplification adjustment to improve speech clarity is not an optimal way of doing it, because you’d then pretty much have to amplify pretty much the whole spectrum. There’s no one narrow sweet spot just for speech only.
If this could be done like you think, then speech clarity is not such a tough problem after all.
The name of the game is not really to find the sweet frequencies to amplify (because there’s really none, it’s all spread out). The name of the game for speech clarity is to improve the signal to noise ratio (SNR) between the speech signal and the noise. That’s why HA mfgs have to tackle the issues in many different ways, from static noise reduction to directionality for noise blocking and better speech focus, to finding other ways to clean up the speech using dynamic noise modeling to apply noise cancellation principal to speech, etc.
So instead of focusing on trying to find a sweet spot on the frequency spectrum for speech, if you’re a DIY with self programming, try to learn all the SNR improvement techniques your hearing aid model has available, and tweak your programming around those techniques.