Automagically scale Tikz-Picture and use Externalization and setting of file name

I am trying to implement the solution to a question: Automagically scale Tikz-Picture AND use Externalization

The answer helps and I managed to do this, however I would like to give each externalized output image a specific file name, e.g. if my tikz file is at path/to/file.tikz then I would like to have the output file at /ext-tikz/path/to/file.pdf.

However, as soon as I implement tikzsetnextfilename{...} the build fails.

Here is a MWE:

documentclass{article}%
%
usepackage{tikz}
usetikzlibrary{external}
tikzexternalize
% 
% from https://tex.stackexchange.com/a/29145/221484
usepackage{environ}
makeatletter
deftikzwidth{textwidth}
deftikzheight{textheight}
newsavebox{measure@tikzpicture}
NewEnviron{tikzsize}[2]{%
deftikzscale{1}%
tikzifexternalizingnext{%
    deftikz@width{#1}%
    deftikz@height{#2}%
    begin{lrbox}{measure@tikzpicture}%
    tikzset{external/export next=false,external/optimize=false}% force translation of this BODY (and do not optimize it away as it would usually do):
    BODY
    end{lrbox}%
    pgfmathparse{#1/wdmeasure@tikzpicture}%
    edeftikzscalewidth{pgfmathresult}%
    pgfmathparse{#2/htmeasure@tikzpicture}%
    edeftikzscaleheight{pgfmathresult}%
    pgfmathparse{min(tikzscalewidth, tikzscaleheight)}%
    edeftikzscale{pgfmathresult}%
    BODY
}{% this will re-use an existing external graphics:
    BODY
}
}
makeatother
% 
%  
begin{document}
% 
% tikzsetnextfilename{output} %<-- if activated it fail's
% 
begin{tikzsize}{tikzwidth}{tikzheight}
begin{tikzpicture}[scale=tikzscale]
draw (0,0)--(1,1);
end{tikzpicture}
end{tikzsize}
% 
end{document}

I noteced inside the log files, that with the activated tikzsetnextfilename it says

A tikzpicture has been optimized away. Use ‘/tikz/external/optimize=false’ to disable this.

However when I use this feature, the generated image is not scaled anymore.