Combining 800 csv files in one table but each file has different number of headers and header titles

I tried to combine 800 cav files via power query although Im not familiar with it. I tried the combine and transform as well as the combine and load but to no avail. The resulting table only contains the columns with the most common headers that can be found across all 800 cav files.

For reference, my csv files have around 10 or 12 header columns that are common while the succeeding column headers may be different in name and in order. the first 12 contains the budget details of each company, department, employee position. There are around 400 cost element headers available and the files only contains the cost elements that they have a budget to.

Thank you!