Vertical space above align (not surrounded by text)

Question

So I know of the fact that you should never begin a paragraph with display math if you want to avoid that weird space above your math. But in my case I have a box, and inside that box I want my equations to be aligned. And align* is display math. And as my box is for equations only, I do not have text above that display math. Hence the vertical space.

How do I align my equations – that are not supposed to be surrounded by text – without having an annoying vertical space above?

\documentclass[a5paper,12pt,dvipsnames,twoside,openany]{memoir}
\usepackage[a5paper]{geometry}
\usepackage[dvipnames,table]{xcolor}
\usepackage[many]{tcolorbox}
\usepackage{tikz}
\usepackage{bm,amsmath,amssymb,amsthm,gensymb,esdiff,siunitx}

\usetikzlibrary{tikzmark,fit,shadows}

\tikzset{highlight/.style args={#1}{fill=#1,
                                    rounded corners=1.6pt,
                                    inner xsep=1ex,
                                    minimum height=1.5em}}

\tcbset{enhanced,
        boxrule=0.8pt,
        bottomrule=1.6pt,
        arc=1pt,
        auto outer arc,
        lifted shadow={1mm}{-1mm}{2mm}{0.1mm}{gray!60}}

\colorlet{backLightYellow}{red!30!yellow!30!white!30}
\colorlet{framLightYellow}{black!30!yellow!30}
\colorlet{backDarkYellow}{black!30!red!30!yellow}
\colorlet{framDarkYellow}{red!30!yellow!40}

\def \epc {6.241 509 074 460 76 E18}
\def \cpe {1.602 176 634 E-19}

\begin{document}

\begin{tcolorbox}[colback=backLightYellow,colframe=framLightYellow]\small
\begin{tikzpicture}[overlay,remember picture]
\iftikzmark{lhs1}{%
  \node[yshift=3pt,fit=(pic cs:lhs1)(pic cs:rhs1),highlight={backDarkYellow},yshift=-.8pt]{};%
  \node[yshift=3pt,fit=(pic cs:lhs1)(pic cs:rhs1),highlight={framDarkYellow}]{};%
}{\typeout{Please recompile.}}
\iftikzmark{lhs2}{%
  \node[yshift=3pt,fit=(pic cs:lhs2)(pic cs:rhs2),highlight={backDarkYellow},yshift=-.8pt]{};%
  \node[yshift=3pt,fit=(pic cs:lhs2)(pic cs:rhs2),highlight={framDarkYellow}]{};%
}{\typeout{Please recompile.}}
\end{tikzpicture}
\begin{align*}
    \tikzmark{lhs1}\SI{\cpe}{\coulomb}
 &= \SI{1}{\elementarycharge}\tikzmark{rhs1}\\
    \frac{\SI{\cpe}{\coulomb}}{\num{\cpe}}
 &= \frac{\SI{1}{\elementarycharge}}{\num{\cpe}}\\
    \tikzmark{lhs2}\SI{1}{\coulomb}
 &= \SI{\epc}{\elementarycharge}\tikzmark{rhs2}
\end{align*}
\end{tcolorbox}

\end{document}