Tighter-spaced bulleted list should have ordinary line spacing to paragraphs/lines above and below

To get tighter line spacing (my document's main spacing is 1.5-spacing, and I'm using memoir), I normally use a self-defined "tight list" (tightlist in the code below). This works well. However I would like the list's spacing to the text above and below to be identical to the ordinary (here: 1.5) line spacing, that is I would like the list to behave like a vertical line-like atom with respect to the rest of the document.
In the following document
\documentclass{memoir}

\OnehalfSpacing

\newenvironment{mytightlist}[3]{
  % noticeably tighter than the memoir class's tightlist facilities
  \begin{Spacing}{1}%
  \begin{list}{#1}{%
    \setlength{\topsep}{0pt}%
    \setlength{\parskip}{0pt}%
    \setlength{\parsep}{0pt}%
    \setlength{\itemsep}{0pt}%
    \setlength{\leftmargin}{#2}%
    \setlength{\labelwidth}{#3}%
    \setlength{\labelsep}{0.5em}%
  }%
}%
{\end{list}\end{Spacing}}


\begin{document}

Text. HHHHHHHHHH

Text. HHHHHHHHHH

\begin{mytightlist}{\(\bullet\)}{1.5em}{0.5em}
\item Text. HHHHHHHHHH
\item Text. HHHHHHHHHH
\end{mytightlist}

Text. HHHHHHHHHH

Text. HHHHHHHHHH

\end{document}
the spacing within line pairs 2/3 and 4/5 is slightly less than the intended spacing, namely that found within line pairs 1/2 and 5/6. Why? How can one fix this?
Btw, I don't care about whether a list "belongs" to a paragraph around it: for me, such spacing should be uniform. ("LaTeX goes to a lot of effort to distinguish in-paragraph lists from end- or start-of-paragraph lists"; comment (quoted approximately) by David Carlisle)
Using the MySpacing environment from this answer to a related question about spacing around blocks instead of Spacing doesn't do the trick in this context.