Placement of logo and putting text on it in scrlttr2

I'm not sure I understand your question. Coding a new letterhead isn't that easy. Maybe we can ignore the idea of a "firsthead" (firsthead=off) and put the elements on the page manually.

\documentclass[firsthead=off]{scrlttr2}

\usepackage{graphicx}
\usepackage{lipsum, picture, calc, booktabs}

% header picture on first page

\newlength{\xlength}
\newlength{\ylength}
\setlength{\xlength}{20mm}
\setlength{\ylength}{-40mm}

\usepackage{eso-pic}
\AddToShipoutPictureBG*{%
  \AtPageUpperLeft{%
    \put(\xlength, \ylength){
      \includegraphics[width=\textwidth + 20mm,height=1in]{example-image-a}}
    \put(\xlength,.5\ylength){
                  \scriptsize\textbf{~This is my logo}
                }
                \put(\xlength,\ylength+2\baselineskip){%
                  \begin{tabular}{ll}\toprule
                    Even a & tabular\\
                    may & follow!\\\bottomrule
                  \end{tabular}
                }
  }
}

\addtoplength{firstheadvpos}{-.3\ylength}
\addtoplength{locvpos}{5cm}

\setkomavar{fromname}{Aaron Berentsen}
\setkomavar{fromaddress}{Ceestreet 1\\Deetown}
\setkomavar{fromphone}{+12345678}
\setkomavar{fromfax}{+12345679}
\setkomavar{fromemail}{[email protected]}

\setkomavar{location}{%
        \scriptsize
        \begin{tabular}{ll}
        \usekomavar*{fromphone}&\usekomavar{fromphone}\\
        \usekomavar*{fromfax}&\usekomavar{fromfax}\\
        \usekomavar*{fromemail}&\usekomavar{fromemail}\\[1em]
        \end{tabular}
}



% sender


% date

\setkomavar{date}{\today}

% subject

\setkomavar{subject}{Subject}


\begin{document}
\begin{letter}{Addressee}

    \opening{Dear Addresse,}
    \lipsum[1]
    \closing{Best wishes,}

\end{letter}
\end{document}

Not sure I fully understand the issue, but you can use \stackinset to overlay text on the image.

\documentclass{scrlttr2}

\usepackage{graphicx}
\usepackage{lipsum,stackengine}

% header picture on first page

\usepackage{eso-pic}
\AddToShipoutPictureBG*{%
  \AtPageUpperLeft{%
    \raisebox{-\height}{%
    \hspace*{1in}%
    \stackinset{r}{.8in}{c}{0in}{\parbox{1in}{\scriptsize
              \usekomavar*{fromphone}\usekomavar{fromphone}\\
        \usekomavar*{fromfax}\usekomavar{fromfax}\\
        \usekomavar*{fromemail}\usekomavar{fromemail}}}{%
    \stackinset{l}{1in}{c}{0in}{\parbox{1in}{\scriptsize\bfseries
      THIS\\IS\\MY\\LOGO}}{%
    \includegraphics[width=\textwidth,height=1in]{example-image-a}%
    }}%
    }%
  }%
}

\setkomavar{firsthead}{}


% sender

\setkomavar{fromname}{Aaron Berentsen}
\setkomavar{fromaddress}{Ceestreet 1\\Deetown}
\setkomavar{fromphone}{+12345678}
\setkomavar{fromfax}{+12345679}
\setkomavar{fromemail}{[email protected]}

% date

\setkomavar{date}{\today}

% subject

\setkomavar{subject}{Subject}


\begin{document}
\begin{letter}{Addressee}

    \opening{Dear Addresse,}
    \lipsum[1]
    \closing{Best wishes,}

\end{letter}
\end{document}

I would use package scrlayer and define a new page style that positions the picture at the desired place using pseudolengths:

\usepackage{scrlayer}
\DeclareNewLayer[
  background,
  voffset=\useplength{firstheadvpos},
  hoffset=\useplength{toaddrhpos},
  width=\useplength{firstheadwidth},
  contents={\includegraphics[width=\layerwidth,height=1in]{example-image-a}}
]{firstheadpicture}
\DeclareNewPageStyleByLayers{firstpage}{firstheadpicture}

The first letter page uses page style empty by default. I would patch \opening to overwrite this default page style:

\usepackage{xpatch}
\xapptocmd\opening{\thispagestyle{firstpage}}{}{\PatchFailed}

fromemail etc. should go to the location field:

\setkomavar{location}{\scriptsize
  \usekomavar*{fromphone}\usekomavar{fromphone}\\
  \usekomavar*{fromfax}\usekomavar{fromfax}\\
  \usekomavar*{fromemail}\usekomavar{fromemail}
}
\setplength[.2]{locwidth}{\useplength{firstheadwidth}}% <- change the width of location to the desired value

Example:

\documentclass{scrlttr2}

%% to show the position of first head and location:
  %\LoadLetterOption{visualize}
  %\showfields{head,location}

\usepackage{graphicx}
\usepackage{lipsum}% only for dummy text

% picture in the background of first head
\usepackage{scrlayer}
\DeclareNewLayer[
  background,
  voffset=\useplength{firstheadvpos},
  hoffset=\useplength{toaddrhpos},
  width=\useplength{firstheadwidth},
  contents={\includegraphics[width=\layerwidth,height=1in]{example-image-a}}
]{firstheadpicture}
\DeclareNewPageStyleByLayers{firstpage}{firstheadpicture}

\usepackage{xpatch}
\xapptocmd\opening{\thispagestyle{firstpage}}{}{\PatchFailed}

% first head
\setkomavar{firsthead}{%
  \vskip .5cm%
  \noindent\hspace*{3cm}%
    \parbox{3cm}{\scriptsize\bfseries
      THIS\\[.2em]
      IS\\[.2em]
      MY\\[.2em]
      LOGO
    }
}

% location
\setkomavar{location}{\scriptsize
  \usekomavar*{fromphone}\usekomavar{fromphone}\\
  \usekomavar*{fromfax}\usekomavar{fromfax}\\
  \usekomavar*{fromemail}\usekomavar{fromemail}
}
\setplength[.2]{locwidth}{\useplength{firstheadwidth}}% <- change the width of location to the desired value

% sender
\setkomavar{fromname}{Aaron Berentsen}
\setkomavar{fromaddress}{Ceestreet 1\\Deetown}
\setkomavar{fromphone}{+12345678}
\setkomavar{fromfax}{+12345679}
\setkomavar{fromemail}{[email protected]}

% date
%\setkomavar{date}{\today}% default

\begin{document}
\begin{letter}{Addressee}
\setkomavar{subject}{Subject}% maybe you write more than one letter with different subjects in this document
\opening{Dear Addresse,}
  \lipsum
  \closing{Best wishes,}
\end{letter}
\end{document}

---

You could use

 \LoadLetterOption{visualize}
 \showfields{head,location}% address, foot and refline are also possible

in the preamble to show the position of head and location.