The first page of scrlttr2 resp. scrletter is somehow special. For example the position of the footer does not only depend on the layout either set using typearea or geometry, but also on the pseudo length firstfootvpos. You can visualize that position using
\LoadLetterOption{visualize}
\showfields{foot}
As already explained in Schweinebacke's answer to: How to avoid empty space at the end of a page when using scrlttr2? the larges text area on the first page can be achieved using option firstfoot=false,enlargefirstpage.
AnotherOption firstfoot=false more or less moves the footer below the page.
Option enlargefirstpage even enlarges the text area, below the layout position of the footer (set by geometry in your case). So the bottom margin is reduced to \footskip on the first page.
But IMHO both are not needed and not wanted in your case. firstfoot=false is more or less accidentally a workaround.
The main problem with your code is, that loading geometry changes the layout parameters but does not automatically adjust the depending pseudo lengths. These are already setup by loading the default DIN.lco, where you can find:
\setplength{firstfootvpos}{1in}%
\addtoplength{firstfootvpos}{\topmargin}%
\addtoplength{firstfootvpos}{\headheight}%
\addtoplength{firstfootvpos}{\headsep}%
\addtoplength{firstfootvpos}{\textheight}%
\addtoplength{firstfootvpos}{\footskip}%
So firstfootvpos depends on not only one but several layout lengths, which geometry sets depending on the font size. So it is not unexpected, that changing the font size and loading geometry without re-loading the letter option DIN.lco results in somehow strange settings.
So the first step to get valid result, would be to add
\LoadLetterOption{DIN}% or whatever letter option you are using
after loading geometry:
\documentclass[a4paper,10pt,version=last]{scrlttr2}%,fontsize=10pt
\usepackage{fontspec}% for lualatex
\usepackage{geometry}
\usepackage[latin]{babel} % for lipsum
\usepackage{lipsum}
\usepackage{showframe}
\setkomavar{subject}[]{\the\dimexpr\useplength{firstfootvpos}\relax}
\LoadLetterOption{DIN}% Load it again after changing the layout by loading geometry
\LoadLetterOption{visualize}
\showfields{foot}
%\KOMAoptions{enlargefirstpage}
%\KOMAoptions{firstfoot=false}
\begin{document}
\begin{letter}{%
CapMini GmbH \\
Egon Walther\\
Walthershofener Straße 20\\
88444 Walthershofen\\
}
\opening{Salve Egon,}
\lipsum{1}
\closing{Regards}% IMHO \\[2\baselineskip] is nonsense here, so I've removed it
\end{letter}
\end{document}
And now, the result without option 10pt or fontsize=10pt and therefore the default 12pt, is very similar:
