You are right that wage and salary are comoletely different and non-interchangeable. So, you can talk about monthly compensation or pay.

Wage is typically referring to a hourly pay, or wages over a period of time. Salary typically refers to pay that is not dependent on how many hours worked. Someone who is paid a salary usually receives the same pay week to week. Other terms are pay, compensation, income.

Words that can replace both in most cases are earnings and income - monthly earnings/income

Actually, ‘wage’ and ‘salary’ can be synonyms in certain contexts, including the one you give (a discussion of economics) That is, people who are paid a salary receive ‘wages’. It’s the more technical term. So it would be natural to talk about monthly or annual wages, even though they probably aren’t paid at that frequency. In ordinary speech, someone paid a higher amount wouldn’t use ‘wage’ to describe their income. He earns $150,000/150k a year. His (annual) salary is $150k. (Annual would usually be understand) His base rate is $22 an hour but with overtime makes about $70,000 a year. He gets a lot of hours. (We wouldn’t say he has a ‘salary’. Honestly, in speech we wouldn’t use the word ‘wage’ that often either. It’s understood from the context.)