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A combination of the options enlargefirstpage and firstfoot=false is a very easy solution to enlarge the first page:

\documentclass[12pt,fromalign=right,fromrule=afteraddress,
               parskip=half,pagenumber=no,DIV=15,
               enlargefirstpage,firstfoot=false]{scrlttr2}

I've removed option foldmarks=true, because it's default. All other options are unchanged from the question.

To see the difference of using the two options as show above, compare the left and right picture:

The left one has been made with:

\documentclass[
  12pt,fromalign=right,fromrule=afteraddress,
  parskip=half,pagenumber=no,DIV=15,
  enlargefirstpage,firstfoot=false
]{scrlttr2}
\usepackage{blindtext}
\begin{document}
\begin{letter}{You\\There}
  \opening{Hey!}
  \blindtext[5]
  \closing{Bye}
\end{letter}
\end{document}

the right one after removing line

  enlargefirstpage,firstfoot=false

BTW: Even

  enlargefirstpage,firstfoot=yes

would result in a little bit more text on the first page:

so the comment, that firstfoot=yes would result in errors is not true. However, if you really want more text on the first page, I would recommend to use firstfoot=false as originally stated.

A combination of the options enlargefirstpage and firstfoot=false is a very easy solution to enlarge the first page:

\documentclass[12pt,fromalign=right,fromrule=afteraddress,
               parskip=half,pagenumber=no,DIV=15,
               enlargefirstpage,firstfoot=false]{scrlttr2}

I've removed option foldmarks=true, because it's default. All other options are unchanged from the question.

To see the difference of using the two options as show above, compare the left and right picture:

The right one has been made with:

\documentclass[
  12pt,fromalign=right,fromrule=afteraddress,
  parskip=half,pagenumber=no,DIV=15,
  enlargefirstpage,firstfoot=false
]{scrlttr2}
\usepackage{blindtext}
\begin{document}
\begin{letter}{You\\There}
  \opening{Hey!}
  \blindtext[5]
  \closing{Bye}
\end{letter}
\end{document}

the left one without line

  enlargefirstpage,firstfoot=false

BTW: Even

  enlargefirstpage,firstfoot=yes

would result in a little bit more text on the first page:

so the comment, that firstfoot=yes would result in errors is not true. However, if you really want more text on the first page, I would recommend to use firstfoot=false as originally stated.

A combination of the options enlargefirstpage and firstfoot=false is a very easy solution to enlarge the first page:

\documentclass[12pt,fromalign=right,fromrule=afteraddress,
               parskip=half,pagenumber=no,DIV=15,
               enlargefirstpage,firstfoot=false]{scrlttr2}

I've removed option foldmarks=true, because it's default. All other options are unchanged from the question.

A combination of the options enlargefirstpage and firstfoot=false is a very easy solution to enlarge the first page:

\documentclass[12pt,fromalign=right,fromrule=afteraddress,
               parskip=half,pagenumber=no,DIV=15,
               enlargefirstpage,firstfoot=false]{scrlttr2}

I've removed option foldmarks=true, because it's default. All other options are unchanged from the question.

To see the difference of using the two options as show above, compare the left and right picture:

The left one has been made with:

\documentclass[
  12pt,fromalign=right,fromrule=afteraddress,
  parskip=half,pagenumber=no,DIV=15,
  enlargefirstpage,firstfoot=false
]{scrlttr2}
\usepackage{blindtext}
\begin{document}
\begin{letter}{You\\There}
  \opening{Hey!}
  \blindtext[5]
  \closing{Bye}
\end{letter}
\end{document}

the right one after removing line

  enlargefirstpage,firstfoot=false

BTW: Even

  enlargefirstpage,firstfoot=yes

would result in a little bit more text on the first page:

so the comment, that firstfoot=yes would result in errors is not true. However, if you really want more text on the first page, I would recommend to use firstfoot=false as originally stated.

Combination of option enlargefirstpage and firstfoot=false is a very easy solution to enlarge first page:

\documentclass[12pt,fromalign=right,fromrule=afteraddress,
               parskip=half,pagenumber=no,DIV=15,
               enlargefirstpage,firstfoot=false]{scrlttr2}

I've removed option foldmarks=true, because it's default. All other options are unchanged from question.

A combination of the options enlargefirstpage and firstfoot=false is a very easy solution to enlarge the first page:

\documentclass[12pt,fromalign=right,fromrule=afteraddress,
               parskip=half,pagenumber=no,DIV=15,
               enlargefirstpage,firstfoot=false]{scrlttr2}

I've removed option foldmarks=true, because it's default. All other options are unchanged from the question.