[P8 Lite (2017) Development] hopeless_bricker:
I also flashed the official su addon from the lineageOS team. Here are the results:

adb shell -> su:

david@punisher   ~  _ adb shell  1 ↵  792  03:50:55
[sudo] password for david:
HWPRA-H:/ $ su
HWPRA-H:/data/data # ls -a
.
..
android
android.ext.services
android.ext.shared
com.android.backupconfirm
com.android.bips
com.android.bluetooth
com.android.bluetoothmidiservice
com.android.bookmarkprovider
com.android.calllogbackup
com.android.camera2
com.android.captiveportallogin
com.android.carrierconfig
com.android.carrierdefaultapp
com.android.cellbroadcastreceiver
com.android.certinstaller
com.android.chrome
com.android.companiondevicemanager
com.android.contacts
com.android.cts.ctsshim
com.android.cts.priv.ctsshim
com.android.defcontainer
com.android.development
com.android.dialer
com.android.documentsui
com.android.dreams.basic
com.android.dreams.phototable
com.android.egg
com.android.email
com.android.emergency
com.android.externalstorage
com.android.facelock
com.android.gallery3d
com.android.htmlviewer
com.android.inputdevices
com.android.keychain
com.android.location.fused
com.android.managedprovisioning
com.android.messaging
com.android.mms.service
com.android.mtp
com.android.pacprocessor
com.android.phone
com.android.printservice.recommendation
com.android.printspooler
com.android.providers.blockednumber
com.android.providers.calendar
com.android.providers.contacts
com.android.providers.downloads
com.android.providers.downloads.ui
com.android.providers.media
com.android.providers.settings
com.android.providers.telephony
com.android.providers.userdictionary
com.android.proxyhandler
com.android.server.telecom
com.android.settings
com.android.sharedstoragebackup
com.android.shell
com.android.smspush
com.android.statementservice
com.android.stk
com.android.storagemanager
com.android.systemui
com.android.terminal
com.android.vending
com.android.vpndialogs
com.android.wallpaper.livepicker
com.android.wallpaperbackup
com.android.wallpapercropper
com.android.wallpaperpicker
com.appgenix.bizcal
com.aspiro.tidal
com.bistudio.minidayz
com.fixeads.olxportugal
com.google.android.apps.maps
com.google.android.apps.photos
com.google.android.apps.turbo
com.google.android.backuptransport
com.google.android.calculator
com.google.android.calendar
com.google.android.deskclock
com.google.android.feedback
com.google.android.gm
com.google.android.gms
com.google.android.gms.policy_sidecar_o
com.google.android.googlequicksearchbox
com.google.android.gsf
com.google.android.inputmethod.latin
com.google.android.instantapps.supervisor
com.google.android.launcher
com.google.android.onetimeinitializer
com.google.android.packageinstaller
com.google.android.partnersetup
com.google.android.setupwizard
com.google.android.syncadapters.contacts
com.google.android.talk
com.google.android.tts
com.keramidas.TitaniumBackup
com.mixplorer
com.orgzly
com.rubenmayayo.reddit
com.samruston.weather
com.teslacoilsw.launcher
com.tritoninteractive.remuda.lite
com.urbandroid.sleep
demigos.com.mobilism
io.github.hidroh.materialistic
keepass2android.keepass2android
lineageos.platform
me.phh.treble.overlay.navbar
me.phh.treble.overlay.nightmode
me.phh.treble.overlay.systemui.falselocks
me.phh.treble.overlay.telephony.lte
me.phh.treble.overlaypicker
net.nullsum.freedoom
org.fdroid.fdroid
org.kde.kdeconnect_tp
org.lineageos.audiofx
org.lineageos.eleven
org.lineageos.lineageparts
org.lineageos.lineagesettings
org.lineageos.lockclock
org.lineageos.profiles
org.lineageos.recorder
org.lineageos.setupwizard
org.lineageos.updater
org.lineageos.weather.provider
org.mozilla.firefox_beta
org.schabi.newpipe
org.thoughtcrime.securesms
projekt.substratum
pt.sibs.android.mbway
HWPRA-H:/data/data # cd ..
HWPRA-H:/data # cd ..
HWPRA-H:/ # ls -a
. dev init.usb.rc root
.. dload init.zygote32.rc rootscan.conf
3rdmodem etc init.zygote64_32.rc sbin
3rdmodemnvm fstab.zram1024m log sdcard
3rdmodemnvmbkp fstab.zram1280m mnt sec_storage
acct fstab.zram1536m mnvm2:0 splash2
bugreports fstab.zram2240m modem_fw storage
cache fstab.zram256m modem_log sys
charger fstab.zram512m odm system
config fstab.zram768m oem ueventd.rc
cust hw_odm patch vendor
cust_comm hw_oem patch_hw verity_key
cust_spec init proc version
d init.environ.rc product version.prop
data init.rc res
default.prop init.usb.configfs.rc resetFactory.cfg
HWPRA-H:/ #

In the developer options menu I'm able to select the "apps and adb" option now and the option "manage root" is enabled as well, but when I launch an app that uses root (TitaniumBackup or mixplorer) the option to give it root access doesn't show up and the app doesn't get access.

What a shame, life without root isn't the same...