math mode - subscripts in upright font by default

Question

The majority of math subscripts I am writing are not variables, but rather names (it's engineering). It is therefore appropriate to typeset them upright. My code would look much, much cleaner, if the default behaviour would be that

$a_m$

typesets the 'm' upright, while for the few cases where I need the normal italic font, I could use something like

$a_\mathnormal{m}$

To clarify: I am not looking for a macro that makes it a bit less cumbersome to typeset an index upright. I already have that. Instead, I want to change/overwrite the default behaviour (in the preamble).

Also, if you want to tell me that I should not change the default behaviour, consider me already notified.

Related questions (none of them provide the answer I am looking for):

Is there a way to make math mode subscripts automatically non-italic?

Typeset subscript material automatically in upright font shape

How to globally put subscripts in math-mode in upright font using amsmath?

Answer 1

David Carlisle pointed me to this answer in one of the linked questions. It does not immediately work with the scrartcl class, as it then produces the error "undefined old font command '\rm'." So it has to be changed to a font that is recognized:

\documentclass{scrartcl}
\usepackage{amsmath}

% typesetting indexes upright by default
\def\subinrm#1{\sb{\textnormal{#1}}}
{\catcode`\_=13 \global\let_=\subinrm}
\mathcode`_="8000
\def\upsubscripts{\catcode`\_=12 } \def\normalsubscripts{\catcode`\_=8 }
% the toggle for upright subscripts
\upsubscripts
% the toggle for italic subscripts
%\normalsubscripts

\begin{document}
    $A_{läke}$, $\normalsubscripts \sum_{i=1}^\infty {1\over n}$
\end{document}

\def\subinrm#1{\sb{\mathrm{#1}}} was my first attempt, but that does not support accented characters such as ä,ö,ü, which, while not pretty, might still show up in my German document. With the amsmath package, \textnormal works in the intended way: Umlauts can be used and subscripts are correctly scaled.

Credit should go to wipet for his original solution and David Carlisle for pointing me to it.

EDIT: updated MWE.