Infering the size of a minipage from the containin

This is a follow-up question to Using bullets list in tikz's node label in rmarkdown. I had some TikZ code that works fine in pure LaTex but NOT when I transport it to rmarkdown where the error ! LaTeX Error: Something's wrong--perhaps a missing \item. is raised. This was solved in the answer to Using bullets list in tikz's node label in rmarkdown but another issue arises applying the solution I got in there.

You can refer to the original question (Using bullets list in tikz's node label in rmarkdown) but basically I have some TikZ code for pictures to be used as part of a larger rmarkdown file. It works in LaTex as I tested in on https://www.overleaf.com/ but once in rmarkdown, it raises the missing item error. The proposed solution in Using bullets list in tikz's node label in rmarkdown was to add a \minipage environment in rmarkdown (see the code below).

My problem with the use of the \minipage environment is that I will have to manually set its width (or at least I don't know how to automate this) before creating the node which is supposed to be part of a large TikZ picture. In other words, I need to know the allocated space for each node to reproduce the picture in rmarkdown. I was wondering whether there is a way to infer the size of the node in advance, so that I can create a minipage matching the size of the node it will contain.

\documentclass{article}

\usepackage{tikz}
\usepackage{enumitem}

\begin{document}
\definecolor{BulletsColor}{rgb}{0, 0, 0.9}
\newlist{myBullets}{itemize}{1}

\setlist[myBullets]{
  label=\textcolor{BulletsColor}{\textbullet},
  leftmargin=*,
  topsep=0ex,
  partopsep=0ex,
  parsep=0ex,
  itemsep=0ex,
  before={\color{BulletsColor}\itshape}
}


\begin{tikzpicture}
  \node[draw, rounded corners] (a)  {
    \begin{minipage}{2.5cm}
      p
      \begin{myBullets}
      \item first item
      \item second item
      \end{myBullets}
    \end{minipage}
  }
  ;
  \end{tikzpicture}
\end{document}

I am also open to other solutions as long as I will NOT have to specify the size of my nodes manually. For example doing (note the commented lines)

\begin{tikzpicture}
  \node[draw, rounded corners] (a)  {
    % \begin{minipage}{2.5cm}
      p
      \begin{myBullets}
      \item first item
      \item second item
      \end{myBullets}
    % \end{minipage}
  }
  ;
  \end{tikzpicture}

in TikZ will infer the size of the node from its text size and I am looking for something that allows me to use the same code in rmarkdown without having to manually specify the size of each minipage across my nodes.

You could replace minipage with the varwidth environment from the package of the same name:

\documentclass{article}

\usepackage{tikz}
\usepackage{enumitem}

\usepackage{varwidth}


\begin{document}
\definecolor{BulletsColor}{rgb}{0, 0, 0.9}
\newlist{myBullets}{itemize}{1}

\setlist[myBullets]{
  label=\textcolor{BulletsColor}{\textbullet},
  leftmargin=*,
  topsep=0ex,
  partopsep=0ex,
  parsep=0ex,
  itemsep=0ex,
%  before={\color{BulletsColor}\itshape}
}


\begin{tikzpicture}
  \node[draw, rounded corners, font=\itshape, text=BulletsColor] (a)  {
    \begin{varwidth}{\textwidth}
      p
      \begin{myBullets}
      \item \textcolor{BulletsColor}{first item}
      \item \textcolor{BulletsColor}{second item}
      \end{myBullets}
    \end{varwidth}
  }
  ;
  \end{tikzpicture}
\end{document}