Apple has had an "increase contrast" option on iOS, which does exactly what you're asking for, specifically designed for a11y, within the Accessibility Settings

Apple has had an "increase contrast" option on iOS, which does exactly what you're asking for, specifically designed for a11y, within the Accessibility Settings panel of iOS, since iOS 8 I believe, potentially earlier. This increases contrasts throughout the entire iOS experience, and yes, makes the green bubbles several shades darker. They are not "inactive". (It's also important to remember that the "green bubbles" applies to messages sent, not received. Received texts, no matter to mechanism, are either black text on grey, or white text on near-black depending on Dark mode)