Go to accessibility settings and enable increased contrast.

Thanks, I tried that and it works! I use dark mode and now instead of a black background with lime green bubbles and white text, it's darker green (and blue) bubbles with white text. Both are easier to read! Functionality is more important to me than aesthetics for a phone.

But that affects the entire OS, and makes it quite ugly.

Eh, it's maybe a bit less attractive but not "quite ugly". It mostly seems to take some bright shades (like reds, greens, and blues) and darkens them. It's not like MS's high contrast option (which does indeed make the whole system quite ugly, the perhaps does an even better job than the iPhone of improving visibility).