How to get derived string recognized as column name. List.Last(Table.ColumnNames(...

Dear all,

I´m new to this forum, but a lot of posts could already help me with my Power Query ambition. Thanks´s a lot for that.
Now, for my last problem I couldn´t find a solution (or rather don´t know the right expression to search for).

My goal is to keep the first row of nested tables, which I could solve by this M code:

= Table.AddColumn(#"Expand all", "First row only", each Table.FirstN([Grouped 5], 1))

This got me a column "First row only" holding nested tables each with exact one row, the top row fetched from the nested tables in column "Grouped 5".

So far so good, but now I created my source with a dynamic amount of columns, meaning the last column  - the one to be processed - can be named "Grouped 1", "Grouped 2" up to "Grouped 7".

By testing this M code:

= Table.AddColumn(#"Expand all - dynamically", "Custom", each List.Last(Table.ColumnNames(#"Expand all - dynamically")))

I could verify to strip the name of the last column, e.g. "Grouped 5".

But putting this into the code for receiving the first rows of each nested table in this last column didn´t work. Obviously there´s a mistake in it:

= Table.AddColumn(#"Expand all - dynamically", "First row only", each Table.FirstN(List.Last(Table.ColumnNames(#"Expand all - dynamically")), 1))

Expression.Error: We cannot convert the value ""Grouped 5"" to type Table.

I think the doubled "" are an indication what´s going wrong, but my experience on M code isn´t sufficient to figure this out.
I hope somebody can help me.

Regards,

Stefan