I had something similar a while ago but not dynamic columns.

In my case I was building a temp table from multiple data.

What I did was create the table with all varchar columns plus a rowid.

Insert the column names with rowid=1

Insert the data (cast/convert) with rowid=2

Select the data ordered by rowid.

If you are pivoting with dynamic columns then you will know the column names, you could use the above technique.