Neglected many aspects of knowledge during my PhD research. What's the best strategy to learn and be comfortable in it?
Actually, your experience is probably (nearly) universal. Doing research requires a deep dive not a broad view. But it also gives you skills. Employ those skills to things that seem "interesting" to you and you will fairly naturally expand the horizon. 

In many fields it is impossible to know everything, as there is too much. Math passed that boundary sometime in the early 20th century. 

Take a long view, assuming that you have suitable employment. Pick a topic that you think would be worth a look, for whatever reason and study it as intensively as you can for some period of time. When you are comfortable with what you have learned or you become bored with it, move on to another. 

Earning a doctorate doesn't mean you have learned everything you will ever learn. Many people even radically change fields after earning a PhD. Many people in one field invent another field. Computer science, for example, grew out of math and engineering.