ok, the test was made as following:

i prepare/partition/mount on /mnt a partition
i install on /mnt the system by obarun-intall and picking the openboxS6 configuration folder
i arch-chroot the /mnt folder and i install this exact list of packages :

qt5-base-nosystemd qtwebkit-nosystemd sddm-nosystemd rtkit-nosystemd modemmanager-nosystemd libmbim-nosystemd pulseaudio-nosystemd  libpulse-nosystemd accountsservice-nosystemd media-player-info-nosystemd upower-nosystemd udisks2-nosystemd libatasmart-nosystemd plasma-meta kdebase-lib plasma-workspace

i change the ~/.xinitrc exactly as it :

# !/bin/bash
exec startkde

i exit from the arch-chroot.
i reboot on the fresh install, kde start perflectly.

So, i don't like this kind of issue grrr.

Anyway, i have made two file of package : one before installing the necessary for plasma and the other after the installation of package listed above. This is the diff between these two file

baloo
bluedevil
bluez
bluez-qt
breeze
breeze-gtk
breeze-icons
cdparanoia
cln
convertlit
docbook-xml
docbook-xsl
drkonqi
ebook-tools
enchant
frameworkintegration
gdb
glu
gpgmepp
gst-plugins-base
gst-plugins-base-libs
gstreamer
hspell
hunspell
icu
ilmbase
iso-codes
jansson
kactivities
kactivities-stats
kactivitymanagerd
karchive
kauth
kbookmarks
kcmutils
kcodecs
kcompletion
kconfig
kconfigwidgets
kcoreaddons
kcrash
kdbusaddons
kde-cli-tools
kde-gtk-config
kdebase-lib
kdeclarative
kdecoration
kded
kdelibs
kdelibs4support
kdeplasma-addons
kdesu
kemoticons
kfilemetadata
kgamma5
kglobalaccel
kguiaddons
kholidays
khotkeys
khtml
ki18n
kiconthemes
kidletime
kinfocenter
kinit
kio
kitemmodels
kitemviews
kjobwidgets
kjs
kjsembed
kmenuedit
knewstuff
knotifications
knotifyconfig
kpackage
kparts
kpeople
kpty
krunner
kscreen
kscreenlocker
kservice
ksshaskpass
ksysguard
ktexteditor
ktextwidgets
kuiserver
kunitconversion
kwallet
kwallet-pam
kwayland
kwayland-integration
kwidgetsaddons
kwin
kwindowsystem
kwrited
kxmlgui
kxmlrpcclient
libatasmart-nosystemd
libcanberra
libdbusmenu-qt4
libdbusmenu-qt5
libdmtx
libgit2
libical
libimobiledevice
libkscreen
libksysguard
libmbim-nosystemd
libmm-glib
libndp
libnewt
libnm-glib
libpgm
libplist
libpwquality
libqalculate
libqmi
libqzeitgeist
libsodium
libteam
libusbmuxd
libvisual
libvoikko
libxkbcommon-x11
libxres
libxslt
libxss
libzip
lm_sensors
lmdb
media-player-info-nosystemd
milou
modemmanager-nosystemd
modemmanager-qt
networkmanager
networkmanager-qt
noto-fonts
nss
openal
openexr
openjpeg2
oxygen
phonon-qt4
phonon-qt4-gstreamer
phonon-qt5
phonon-qt5-gstreamer
plasma-desktop
plasma-framework
plasma-integration
plasma-mediacenter
plasma-meta
plasma-nm
plasma-pa
plasma-sdk
plasma-workspace
plasma-workspace-wallpapers
polkit-kde-agent
polkit-qt4
polkit-qt5
poppler
poppler-qt5
powerdevil
prison-frameworks
pulseaudio-nosystemd
qca-qt4
qca-qt5
qrencode
qt4
qt5-base-nosystemd
qt5-declarative
qt5-graphicaleffects
qt5-location
qt5-multimedia
qt5-quickcontrols
qt5-script
qt5-sensors
qt5-svg
qt5-tools
qt5-webchannel
qt5-webkit
qt5-x11extras
qt5-xmlpatterns
qtchooser
qtwebkit-nosystemd
reflector
rtkit-nosystemd
sbc
scrnsaverproto
sddm-kcm
sddm-nosystemd
socat
solid
sonnet
sound-theme-freedesktop
strigi
systemsettings
taglib
tdb
threadweaver
tslib
ttf-oxygen
udisks2-nosystemd
upower-nosystemd
user-manager
webrtc-audio-processing
xcb-util-cursor
xcb-util-image
xcb-util-keysyms
xcb-util-wm
xorg-xmessage
zeromq

please compare it with your system. be sure that all these package exist on your system. diff command is perfect for that

Can you post the log output of Xorg?

just change the variable TZ to reach the good one in your /etc/s6.conf

# Set timezone, availables timezones at /usr/share/zoneinfo.
TZ=Indian/Reunion