Since this awnser grew slowly lets start with the result, the same image once scaled to .3\linewidth and once scaled to \linewidth. The first section talks about the idea while the implementation is at the very bottom.
Source:
image.tikz is at the bottom of this post.
\documentclass[]{scrreprt}
\usepackage{standalone}
\usepackage{tikz}
\usepackage{tikzscale}
\usetikzlibrary{calc}
\begin{document}
\begin{figure}
%\includestandalone[width=0.9\linewidth]{subfiles/TopView}
\includegraphics[width=.3\linewidth]{image}
\caption{Top view}\label{fig:topView}
\end{figure}
\includegraphics[width=\linewidth]{image}
\end{document}
There is a package tikzscale, which does most of waht you ask, from the documentation:
So although the original tikzpicture itself has the width of a
complete line, it gets proportionally scaled down to half the width
while being loaded from the \includegraphics command. Neither the
line’s thickness nor the text center are scaled, [...]
If you use your graphics in documents with different font sizes though, be aware that the units em and ex change, thus their use might get you unwanted results if you mix them with absolute units.
I usually have one file with the headers and the preview package, that I use to render the image standalone. I use input to include the different images in that file.
\documentclass{report}
\usepackage{tikz}
\usepackage[margin=0cm,nohead]{geometry}
\usepackage[active,tightpage]{preview}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc} %input encoding
\usetikzlibrary{...}
\begin{document}
\input{image1}
%\input{image2}
\end{document}
So far I have not come arround to use tikz externalize which would leave you with pdfs automatically and it is supported by tikzscale. This might make my preview wrapper unnecessary.
Note that you do need to render the tikz files for every document you scale them in, since you don't want to scale the font/linewidths.
Scaling
Regarding OPs comment: Scaling as you requested can not work from prerenders, since in a image file all pixels are equal, there is no difference between fonts/linewidth and linelength. The only option to rescale tikz images and to keep fontsize/linesizes I know of is tikzscale. Tikzscale has some very strict requirements, one being that the image is in a file with the extension .tikz. Thus I renamed the second file to end in tikz. I then change your main file to this:
\documentclass[]{scrreprt}
\usepackage{standalone}
\usepackage{tikz}
\usepackage{tikzscale}
\begin{document}
\begin{figure}
%\includestandalone[width=0.9\linewidth]{subfiles/TopView}
\includegraphics[width=.5\linewidth]{image}
\caption{Top view}\label{fig:topView}
\end{figure}
\end{document}
If you now run this you will get an error along the lines: Request to scale an unscalable image.
This comes from the fact that you image IS unscalable. Your image is based on a node with an includegraphics. tikzscale only scales tikz coordinates. You then place nodes at the edges of the unscalable node holding text (text may not scale so these dont scale either).
So how does one obtain a scalable image?
add this as the last line of your picture: \node at(2,2) {scaler};
You can try this method with different tikzpictures, then it should work.
Or as a workarround: Place two coordinates as coornerpoints of a rectangle (best with mesurement in an absolute mesure like cm), calculate the size of this and use scalebox to include the pdf. This way if tikzscale shifts the coordinates your included pdf scales. Therefore your image is now scalable.
here is an implementation of a workarround, It works as follows:
with \pgfgettransformentries we obtain the current scale matrix and use the x or y-scale to scale the included image by using graphicx scale option by using \includegraphics[scale=\a]{image2.pdf}};. Therefore the image size changes and the whole thing can be scaled.
\documentclass[tikz,convert=false]{standalone}
\usepackage{tikz, fp, tikz-3dplot}
\usetikzlibrary{calc, intersections, arrows, fixedpointarithmetic, decorations.markings}
\graphicspath{../images/}
\begin{document}
\begin{tikzpicture}[>=latex,line cap=round]
\pgfgettransformentries{\a}{\b}{\c}{\d}{\xtrans}{\ytrans}
\node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[scale=\a]{image2.pdf}};
\begin{scope}[x={(image.south east)},y={(image.north west)}]
\draw[help lines,xstep=.1,ystep=.1] (0,0) grid (1,1);
\foreach \x in {0,1,...,9} { \node [anchor=north] at (\x/10,0) {0.\x}; }
\foreach \y in {0,1,...,9} { \node [anchor=east] at (0,\y/10) {0.\y}; }
\node at (0.854,0.855) [rotate=-40]{$\vartheta^\prime$};
\node at (0.19,0.27) [rotate=40]{$f$};
\node at (0.055,0.605) [rotate=30]{$\delta$};
\node at (0.53,0.63) [rotate=40]{$R$};
\node at (0.15,0.2) [rotate=-40]{$l_\mathrm{c}$};
\draw[-latex, black] (0.8,0.1) -- (0.8,0.3) node[pos = .5, above, rotate = 90]{$\vec{u}$};
\draw[-latex, blue] (0.1,0.8) -- (0.3,0.8) node[pos = .5, above, rotate = 0]{$\vec{c}_\mathrm{m}$};
\end{scope}
\end{tikzpicture}
\end{document}
