If you are lucky it might be a frayed cable or rusted housing or an "industrial" washout and relube of the brifter
might fix it, but IME 8spd brifters just didn't last very long. You are right, no 7spd brifters. Most likely site of
cable fray is inside the brifter where it winds tightly around the drum.

Microshift makes good quality cheap 8spd: https://www.microshift.com/en/product/sb-r483/