So what's the right way to set the preferred line spacing? Changing default line spacing in Book returns underfull \vbox warnings

Question

Simple task: book-class document, 12pt font size, 6x9 page size. With the default book settings (I don't really know what's the default line spacing in book-class document) the document is typeset perfectly with no warnings. But it returns only 37 lines on page and a quiet wide line spacing for my needs. I'd prefer more lines (maybe 42) without changing the font size or the margins. The only way I see it done is by reducing the default line spacing that (to my liking) is too wide.
I tried all the methods suggested here Line spacing in book document class

but no matter what value I put in the text looks ugly and the code returns a lot of underfull \vbox (badness) warnings. \linespread{} doesn't seem to have an ability to decrease the line spacing, only to increase.
So what's the right (and simple) way to set a value (other than the default one) for the document's line spacing? In my case to decrease it. And preferably only for the \mainmatter so the title and all the pages before the \mainmatter won't get affected. Here's the MWE.

% !TEX TS-program = LuaLaTeX
\documentclass[12pt,twoside,openany]{book}
\pagestyle{plain}
\usepackage[english, russian]{babel}
\usepackage{fontspec}
\setmainfont{EB Garamond}[
]
\usepackage{microtype}
\usepackage[shortcuts]{extdash}
\usepackage[pagewise]{lineno}
\linenumbers 
\usepackage[
  paperwidth=6in,
  paperheight=9in,
  inner=13mm,
  top=15mm,
  outer=20mm,
  bottom=24mm, showframe,
  heightrounded,
]{geometry} 

\usepackage{lettrine}
\setcounter{DefaultLines}{3}
\renewcommand{\DefaultLoversize}{0.1} 
\renewcommand{\DefaultLraise}{0} 
\renewcommand{\LettrineTextFont}{}
\setlength{\DefaultFindent}{\fontdimen2\font}

\usepackage{lipsum}
\setlength{\parskip}{0pt} 
\usepackage{graphicx}
\usepackage[dvipsnames]{xcolor}
\usepackage[pages=some]{background}

\usepackage{fancyhdr}
\fancyhf{} 
\renewcommand{\headrulewidth}{0ex} 
\fancyfoot[LE,RO]{\thepage} 
\pagestyle{fancy}
\fancypagestyle{plain}{%
  \fancyhf{}%
  \renewcommand{\headrulewidth}{0ex}%
  \fancyhf[lef,rof]{\thepage}%
}
 
\usepackage{etoolbox}
\makeatletter
\patchcmd{\chapter}{\if@openright\cleardoublepage\else\clearpage\fi}{\par}{}{}
\makeatother

%\raggedbottom % If used, it messes up the bottom line the way that its baseline hangs above the frame on some pages

\usepackage{titlesec}
\titleclass{\part}{top}
\titleformat{\part}{\vfil\penalty500\vfilneg\centering\normalfont\large}{\thepart.}{4ex}{\LARGE\centering}
\titlespacing*{\part}{0ex}{2ex plus 2ex minus 1ex}{0ex} 
\titleclass{\chapter}{straight}
\titleformat{\chapter}{\vfil\penalty500\vfilneg\centering\normalfont\large}{\thechapter.}{4ex}{\large\centering}
\titlespacing*{\chapter} {0ex}{6ex plus 4ex minus 1ex}{2ex} 

\setcounter{secnumdepth}{0}
\usepackage{tocloft}

\begin{document} 
\pagestyle{plain}
\pagenumbering{gobble}
\addtocounter{page}{0}

\begin{titlepage}
    \centering
    {\large TITLE\par}
    \vfill
    \title{SOME TITLE}
    \author{SOME AUTHOR}
    \date{}
\end{titlepage}
\clearpage
\frontmatter

\mainmatter
\pagenumbering{arabic}

\part{PART ONE}
\begin{center}
    \includegraphics[height=15ex,keepaspectratio]{example-image-a}
  \end{center} 
\chapter{CHAPTER ONE}
\begin{center}
    \includegraphics[height=35ex,keepaspectratio]{example-image-b}
  \end{center} 
\lipsum[1-24]
\chapter{CHAPTER TWO}
\begin{center}
    \includegraphics[height=35ex,keepaspectratio]{example-image-b}
  \end{center} 
\lipsum[1-24]
\chapter{CHAPTER THREE}
\begin{center}
    \includegraphics[height=35ex,keepaspectratio]{example-image-b}
  \end{center} 
\lipsum[1-24]  
\end{document}

Answer 1

Page 2 has only text, so the page is filled by lines, starting \topskip from the top and then extending for n \baselineskip. So you require

\textheight=\topkip + n\baselineskip

for some n. That is what the heightrounded option of geometry does, but if you change baselineskip later you need to re-do the calculation.

For a quick test you can set things right after \begin{document}

\begin{document} 
\setlength\baselineskip{13pt}
\setlength\textheight{\dimexpr\topskip+40\baselineskip}

For example gives no warnings and 41 lines (you could decrease \topskip, or squeeze \baselineskip even more)

For a more complete fix don't set after begin document, modify \normalsize. size12.clo has

\renewcommand\normalsize{%
   \@setfontsize\normalsize\@xiipt{14.5}%
   \abovedisplayskip 12\p@ \@plus3\p@ \@minus7\p@
   \abovedisplayshortskip \z@ \@plus3\p@
   \belowdisplayshortskip 6.5\p@ \@plus3.5\p@ \@minus3\p@
   \belowdisplayskip \abovedisplayskip
   \let\@listi\@listI}

so \renewcommand to have \@setfontsize\normalsize\@xiipt{13} You may also want to reduce some of the other spaces, to match.