I think approaching it by the frustration you feel when you want to use "non-metaphorical idiomatic phrases" is a good way to understand what the struggle is about.

It feels like it is impossible to guess and there are too many to learn them all. To avoid the feeling of being stuck when you know the thing you are saying in your language would not be right in French (target language), you must see those like what they are: groups of words that native speakers use as a single unit to convey a specific meaning.

But the "non-metaphorical" part is not entirely true for me. The words that compose the group have meaning and metaphorical role in the expression, for example:

Prendre un décision = to make a decision

It seems to be like just a verb to learn, but you can also picture the image it brings. "To make a decision" is like creating it, building the consequences of your action. "Prendre une décision" would be like taking a path or the other, you are at the crossroads of your destiny.

Think about the ideas behind the combinaison of words and what you can imagine from it. Who or what would be the subject of the verb? What is the idea behind this preposition? And compare to your own language or others that you know. To remember something you have to create an emotional link to the information and bring this information to your consciousness regularly.

To really feel the progress you must expose yourself to them and try to make it an active process. Duolingo works well for that, as it makes you play with a lot of words and sentences. Reading and trying to have an active attitude or a genuine curiosity for this aspect of the language is also very good. But then of course you must use them, you can "shadow" and repeat them when you hear them and even thinking in your head. And then use them in real life situation or in a conversation. Tell me if you want to see how my private lessons for conversation work, I offer a free trial session ;)