Insert the number of slides up to a final slide in a rmarkdown::beamer_presentation

Question

In a beamer presentation generated with rmarkdown::beamer_presentation, I currently have \insertframenumber/\inserttotalframenumber which shows the current page and the total number of slides in the presentation.

How to insert the number of slides up to a "final slide" in the footer instead of the overall "total number of slides"?

Note: I would like to refrain from having to add {.noframenumbering} to all slides in the appendix.

MWE

Preamble.tex

\setbeamertemplate{footline}{
    \leavevmode%
    \hfill
        \hyperlinkappendixstart{\insertframenumber/\inserttotalframenumber}
}

Presentation.Rmd

---
title: "Slide counter ends at specific slide"
output:
  bookdown::pdf_book:
    base_format: rmarkdown::beamer_presentation
    latex_engine: lualatex
    toc: false
    slide_level: 2
header-includes:
  - \input{files_beamer/preamble}
---

## Slide 1

## Slide 2

## Slide Final
==> count up to this slide

``` {=latex}
\insertframeendpage
```

## Additional Slide 1 (not counted)

## Additional Slide 2 (not counted)

Answer 1

If your beamer version is up-to-date, you can use the \setbeamertemplate{page number in head/foot}[appendixframenumber] template. No need for additional packages.

---
title: "Slide counter ends at specific slide"
output:
  bookdown::pdf_book:
    base_format: rmarkdown::beamer_presentation
    latex_engine: lualatex
    toc: false
    slide_level: 2
    keep_tex: true
header-includes: |
  \makeatletter\beamer@ignorenonframefalse\makeatother

  \setbeamertemplate{page number in head/foot}[appendixframenumber]

  \setbeamertemplate{footline}{%
    \leavevmode%
    \hfill
    \hyperlinkappendixstart{%
      \usebeamertemplate{page number in head/foot}%
    }
  }

---

## Slide 1

## Slide 2

## Slide Final
==> count up to this slide

``` {=latex}
\end{frame}
\appendix
\begin{frame}
\frametitle{Additional Slide 1 (not counted)}
```

## Additional Slide 2 (not counted)

Answer 2

Just add to the YAML-header:

header-includes:
  - \usepackage{appendixnumberbeamer}

(See this SO post).

Alternatively one could likely define a new command using something like \inserttotalframenumber - \insertappendixframenumber somewhere before \setbeamertemplate{footline}{...}.