# Matching difficult formatting of table

I have the following table I'm trying to recreate:

I'm translating the text to English and trying to obtain a similar form, but I'm having a terrible time managing the wild world of LaTeX tables! Here's what I have, or at least the closest I've gotten, which isn't much.

\usepackage{multicol,multirow,tabularx,pbox,makecell}
\usepackage{titling}
\usepackage{lettrine}
\usepackage{soul}

\begin{document}

\noindent
\begin{tabularx}{\textwidth}{||>{\centering\arraybackslash}X|>{\centering\arraybackslash}X|>{\centering\arraybackslash}X|>{\centering\arraybackslash}X||}
\hhline{|t:====:t|}
\multirow{2}{2cm}{\centering\arraybackslash\textsc{RATIO} \newline \footnotesize of the \newline \textsc{\footnotesize radii}.} &
\multicolumn{2}{>{\hsize=\dimexpr2\hsize+2\tabcolsep+\arrayrulewidth\relax\centering\arraybackslash}X|}{\textsc{\footnotesize ratio of the thickness of the electric layer on the two spheres,}} &
\multirow{2}{2.5cm}{\centering\arraybackslash\textsc{DIFFERENCE \footnotesize between calculation} \footnotesize and \textsc{\footnotesize observation.}} \\
\hhline{||~~~~||}
& \footnotesize according to calculation. & \footnotesize according to experiment. & \\
\hhline{||----||}
a & b & c & d \\
a & b & c & d \\
a & b & c & d \\
\hhline{|b:====:b|}
\end{tabularx}
\end{document}


Well evidently it's not going so well. I need a thicker border line, better vertical spacing of the headers, and the flourish I'm not too concerned about, but it would be a nice addition. So what packages can provide a relatively easy path towards this format? And how can I get closer to the goal, with vertical spacing and all?

A place to start from with a NiceTabular nested inside of a tcolorbox:

\documentclass{article}
\usepackage{pbox}
\usepackage{tcolorbox}
\newtcolorbox{mybox}{colback=white,
colframe=black,
boxrule=1.5pt,
sharp corners,
top=0pt,
bottom=0pt,
right=0pt,
left=0pt
}

\usepackage{nicematrix}

\newlength{\tablewidth}
\setlength{\tablewidth}{\dimexpr\textwidth-8.7pt-5\arrayrulewidth}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}

\begin{document}

\noindent
\begin{mybox}
\centering
\begin{NiceTabular}{*{4}{C{0.25\tablewidth-2\tabcolsep}}}[hvlines,cell-space-limits=3pt]