Joining phrases, how to exactly use idioms like "with regard to", "as regard", "in respect"

Solution 1:

I'm not 100% sure these rise to the level of idioms. An idiom is a common fixed expression that must be used with the typical words or it sounds wrong. This limits your word choice. As you've already demonstrated there are many acceptable words that would carry the same meaning and not seem out of place. The only way these sound wrong is if you make a grammatical mistake.

Some definitions of idiom make a big deal about literal vs figurative meanings as well, which would also cast doubt here, but I've always thought that primarily it was their fixed nature that set them apart.

So if they aren't idioms then aren't they just synonyms with varying prepositions? If they mean the same thing, and you follow the rules of grammar, then you can use whichever one you feel like using.

Solution 2:

The phrases using regard tend to refer to an abstract or general object that is the topic or area discussed by the containing sentence or paragraph. Those using respect point to an object that is something more specific than the general context.

In regard to music, he is quite talented. In respect to the oboe, however, he needs more practice.