Lack of sharpness at f1.8 on Canon 50mm STM

Absolute sharpness is generally something you do not want for a portrait. Generally a good portrait shot is a little soft. People usually don't want to see every freckle and skin blemish. For this reason, image processing software which has pre-sets for portraits and landscapes and so on softens the portrait pre-set just as it boosts the saturation on the landscape one.

More broadly, it is good photographic practice to respect the natural limitations of your equipment. It is normal for equipment to function best in the middle of its useful range, less well at the extremes. With lenses, it is normal to have less sharpness and more flare wide open - in fact it is very difficult  to make a lens which doesn't act like this, and although it can be done the result is heavy and expensive. Similarly, zoom lenses very seldom perform especially well at the limits of their zoom range.

Suppose you want to shoot at 35mm and you have two lenses, a 16-35 and a 24-70. Which lens should you use? Well, the 16-35 will do OK, but chances are that it will be a bit soft at 35mm and have some distortion. The 24-70 will be in its sweet spot.

For the same sort of reason, it should be your usual habit to shoot a stop or at least a half stop above wide open. Try to use f/8 on an f/6.3 lens; try to go for f/2 with an f/1.4 lens, and so on.

Note that some lenses are so beautifully made that you can ignore the normal rules. I habitually shoot a Canon 600/4 wide open at f/4 for example, I expect perfect sharpness at f/4, and get it. But it weighs 4 kilograms and cost $15,000.

Finally, there are times when you can't get the shot doing the "right" thing (e.g., shooting stopped down a little, not using outlandishly high ISOs). At these times, you do whatever it takes to get the shot.