How get more pin sharp shots with a lens known for inaccurate autofocus?
The obvious solution, as you have already discovered, is careful manual focusing.
Before you abandon autofocus for manual focus, though, be sure you're not blaming the AF system for user error. Only after you have eliminated improper technique, often caused by not understanding how AF systems operate in general and how different ones operate in particular, can you pin all of the blame on the limitations of the camera or lens.
This answer to How do I diagnose the source of focus problem in a camera? systematically works through all of that and includes a plethora of links to other questions/answers here that discuss each step in detail.
In situations when it is not possible to manually focus, eliminating as much of the error as possible using good techniques and practices covered in many of the links at the answer linked above, as well as understanding how your particular AF system works (here's an answer that surveys one such AF system) , will often go a long way towards improving your results, even with a lens that has a reputation as being inconsistent.
There should probably be a distinction made between accurate focus and consistent or precise focus.
- If a lens/camera system focuses in the same spot every time, within an acceptable amount of variation, it is said to be consistent or precise.
- If a lens/camera system focuses where it is told, within an acceptable amount of variation, it is said to be accurate.
Compare it to using a bow to shoot arrows at a target:
Equate 'consistent' and 'precise' in the image above.
If a system is precise but not accurate, it is referred to as systematic error. If a system is accurate but not precise, it is referred to as random error.
It is possible for a lens/camera system to be consistent without being accurate. Imagine a lens that always focuses the same distance in front of the intended target. In such a case, the systematic error may be corrected using AF Microadjustment (Canon) or AF Fine Tune (Nikon) if the camera in question includes that capability.
In the case of a lens/camera system that is fairly accurate but is not consistent from one shot to the next, there's not a whole lot you can do because the error is random in nature. At best, using AFFT/AFMA will result in half the shots being missed in front of the intended target and the other half being missed behind the target.
No autofocus system is perfect. Some are more consistent/precise than others. As Roger Cicala, founder of lensrentals.com and a well known blogger on such topics, points out in Autofocus Reality Part 3A:¹ the entire system is only as good as the weakest link. To get accuracy and precision you need both a camera and a lens that is capable of both.
¹ For links to the entire Autofocus Reality series, please see this answer to another question.