LaTeX Beamer -- Nested itemize, vertical spacing isn't constant - latex

Question

I'm working on a beamer template and I'm struggling to customize my itemize environment. More precisely, I've troubles with the itemsep in my itemize environment. In general, I'm changing the font size for deeper levels (-> the font size becomes smaller) and that's where the problems begin.
In the appendix you can see a picture with three itemize levels (itemsep_not_constant). If you look closely, then you can see that the vertical spacing between the penultimate and the last items of the same level becomes smaller. This applies for the first level and for the second level.
I've noticed that this problem only occurs, if I add a another (deeper) itemize level with a different font size. Therefore, you can see that the vertical spacing between the items in the third level remains constant.
To get rid of this problem I've tried different approaches, e.g. defining a \newlist. After some research in the internet I've found out that it is recommended to use the \setbeamertemplate for customizations and to avoid using the itemize package (\usepackage{itemize}) in beamer.
Here's my MWE:
\documentclass[aspectratio=169,xcolor=table,12pt]{beamer}
% compile in pdfLaTeX
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[type=none]{fgruler} % ruler
\usepackage{xcolor}
\renewcommand{\rmdefault}{phv}
\renewcommand{\sfdefault}{phv}
\definecolor{footerGrey}{RGB}{135,135,135}
\setbeamercolor*{normal text}{fg=footerGrey}
\setbeamertemplate{itemize/enumerate body begin}{\normalsize}
\setbeamertemplate{itemize/enumerate subbody begin}{\footnotesize}
\setbeamertemplate{itemize/enumerate subsubbody begin}{\scriptsize}
\setbeamertemplate{itemize item}{\color{footerGrey}\rule[0.9mm]{0.6ex}{0.6ex}}
\setbeamertemplate{itemize subitem}{\color{footerGrey}\rule[0.8mm]{0.6ex}{0.6ex}}
\setbeamertemplate{itemize subsubitem}{\color{footerGrey}\rule[0.5mm]{0.6ex}{0.6ex}}
\begin{document}
\begin{frame}
\fgruler{upperleft}{0cm}{0cm}%
\begin{itemize}
\item Item 1
\item Item 2
\item Item 3
\begin{itemize}
\item Item 4
\item Item 5
\item Item 6
\begin{itemize}
\item Item 7
\item Item 8
\item Item 9
\item Item 10
\end{itemize}
\end{itemize}
\end{itemize}
\end{frame}
\end{document}
I'm running out of ideas and I'd really appreciate any help! Thank you and
DIFFERENT APPROACHES THAT HAVE BEEN TRIED:
define \setbeamertemplate (see case description);
another approach:
create a newlist and change the settings for each level
\setlist[unthaItemize,1]{font=\sffamily\bfseries\Huge,
label=\color{footerGrey}\rule[0.9mm]{0.6ex}{0.6ex},
align = left,
leftmargin = 0.025\paperwidth,
labelsep = 0pt,
labelwidth = 8pt,
itemindent= -0.5pt,
topsep = 2.5pt,
itemsep = 2.5pt,
}
\setlist[unthaItemize,2]{label=\color{footerGrey}\rule[0.8mm]{0.6ex}{0.6ex},
align = left,
leftmargin = 0.025\paperwidth,
labelsep = 0pt,
labelwidth = 8pt,
itemindent= -0.5pt,
topsep = 2.5pt,
itemsep = 2.5pt,
%before* = \fontsize{10pt}{10pt}\selectfont
}
etc.

Answer 1

You have to finish the paragraph (e.g. by inserting an empty line) before you switch to a different font size (see also https://github.com/josephwright/beamer/issues/530):
\documentclass[aspectratio=169,xcolor=table,12pt]{beamer}
% compile in pdfLaTeX
\usepackage[T1]{fontenc}
%\usepackage[utf8]{inputenc}
\usepackage[type=none]{fgruler} % ruler
%\usepackage{xcolor}
\renewcommand{\rmdefault}{phv}
\renewcommand{\sfdefault}{phv}
\definecolor{footerGrey}{RGB}{135,135,135}
\setbeamercolor*{normal text}{fg=footerGrey}
\setbeamertemplate{itemize/enumerate body begin}{\normalsize}
\setbeamertemplate{itemize/enumerate subbody begin}{\footnotesize}
\setbeamertemplate{itemize/enumerate subsubbody begin}{\scriptsize}
\setbeamertemplate{itemize item}{\color{footerGrey}\rule[0.9mm]{0.6ex}{0.6ex}}
\setbeamertemplate{itemize subitem}{\color{footerGrey}\rule[0.8mm]{0.6ex}{0.6ex}}
\setbeamertemplate{itemize subsubitem}{\color{footerGrey}\rule[0.5mm]{0.6ex}{0.6ex}}
\begin{document}
\begin{frame}
\fgruler{upperleft}{0cm}{0cm}%
\begin{itemize}
\item Item 1
\item Item 2
\item Item 3
\begin{itemize}
\item Item 4
\item Item 5
\item Item 6
\begin{itemize}
\item Item 7
\item Item 8
\item Item 9
\item Item 10
\end{itemize}
\end{itemize}
\end{itemize}
\end{frame}
\end{document}