This is most likely a lens fault, not normal user error.

Based on the sample descriptions, the blur pattern suggests optical misalignment rather than missed focus or shutter shake. Community replies point to signs such as unusually strong softness, uneven sharpness from one side of the frame to the other, and behavior that gets much worse at the long end. That can happen if an element is decentered or if the image stabilization group has been knocked out of position after a drop or shock.

A very small improvement when stopping down can also fit an optical alignment problem, but it won’t fix it.

Since you’ve already tried a fast shutter speed and a tripod, basic technique is unlikely to be the cause. The practical answer is to have the lens inspected, repaired, or replaced. On a low-cost telephoto zoom, replacement is often the more sensible option than repair.

If you want to double-check, compare shots at different focal lengths with IS on and off, using live view/manual focus on a detailed flat subject. But from the symptoms described, the lens itself is the likely issue.