Based on the discussion, the most likely cause is focus placement rather than a bad camera. If you’re manually focusing all the way to the infinity mark, you may actually be focusing past infinity, and lens distance markings often aren’t precise enough to trust exactly.

A second factor is aperture. Your own tests suggest that with the lens wide open, distant scenes weren’t sharp, and stopping down improved things a lot. That makes sense: distant scenes usually need more depth of field than close-ups. Using a smaller aperture can help, though very small apertures may trade some sharpness for diffraction on some cameras.

What to try:

• Don’t just crank the lens to the infinity stop.
• Focus carefully on a distant subject rather than relying on the scale.
• For landscapes or groups, stop down from wide open and test which aperture gives the best result on your camera/lens.
• Compare shots at several apertures and focus points to find the sharpest combination.

So yes, you can likely improve this without upgrading gear: use more careful distant focusing and avoid shooting far-away scenes wide open unless you’ve verified that focus is accurate.