After factory data reset when you are on stock ROM at androidOS v6.0, do not tie any google account, first install the NetGuard by F-Droid using the mobile built-in browser (not the Chrome). On NetGuard options set to Block all.
at advanced options enable to manage system apps and nothing else
enable only these (in case near PC skip this step)
if we not enable Download Manager & Media Storage, the built-in browser will be unable to save anything we download. Casual error is
At this time, before do anything, we going to Application Manager and disable some useless apps. By default, the system apps is not appearing, make sure you click here
then click each application and press DISABLE button. In some applications this option is grayed out, you cannot do anything. Once an application is DISABLED, the icon disappear from drawer and nobody can call this application. In this step is good to make a device restart.
browse and download the needed..
Browser - Kiwi Browser - 2018 version aka Quenepa - https://downzen.com/en/android/kiwi-browser-fast-quiet/download/quenepa/
RSS Reader - InoReader - https://apkpure.com/inoreader-news-rss-reader/com.innologica.inoreader/download
Maps - Organic Maps - https://github.com/organicmaps/organicmaps - will download world map then you have to search for your country.
Notes - Pocket Plan - https://f-droid.org/en/packages/com.pocket_plan.j7_003/
Music Player - Musicolet - https://krosbits.in/musicolet/
Youtube Player - NewPipe - https://f-droid.org/en/packages/org.schabi.newpipe/
Email - K-9 Mail - https://f-droid.org/packages/com.fsck.k9/
File Manager - Root Explorer - https://apkpure.com/root-explorer/com.speedsoftware.rootexplorer
File Text Editor - QuickEdit - https://apkpure.com/quickedit-text-editor/com.rhmsoft.edit
Barcode Scanner - QuickMark - http://www.quickmark.com.tw/En/basic/downloadMain.asp
find more here

at advanced options enable to manage system apps and nothing else

enable only these (in case near PC skip this step)

for systemOS option - Always-on VPN read the documentation
if we not enable Download Manager & Media Storage, the built-in browser will be unable to save anything we download. Casual error is
<Untitled> Download unsuccessful
At this time, before do anything, we going to Application Manager and disable some useless apps. By default, the system apps is not appearing, make sure you click here

then click each application and press DISABLE button. In some applications this option is grayed out, you cannot do anything. Once an application is DISABLED, the icon disappear from drawer and nobody can call this application. In this step is good to make a device restart.
browse and download the needed..
Browser - Kiwi Browser - 2018 version aka Quenepa - https://downzen.com/en/android/kiwi-browser-fast-quiet/download/quenepa/
once you are with Kiwi, block the built-in browser and Download Manager & Media Storage.
RSS Reader - InoReader - https://apkpure.com/inoreader-news-rss-reader/com.innologica.inoreader/download
Maps - Organic Maps - https://github.com/organicmaps/organicmaps - will download world map then you have to search for your country.
Notes - Pocket Plan - https://f-droid.org/en/packages/com.pocket_plan.j7_003/
Music Player - Musicolet - https://krosbits.in/musicolet/
Youtube Player - NewPipe - https://f-droid.org/en/packages/org.schabi.newpipe/
Email - K-9 Mail - https://f-droid.org/packages/com.fsck.k9/
File Manager - Root Explorer - https://apkpure.com/root-explorer/com.speedsoftware.rootexplorer
File Text Editor - QuickEdit - https://apkpure.com/quickedit-text-editor/com.rhmsoft.edit
Barcode Scanner - QuickMark - http://www.quickmark.com.tw/En/basic/downloadMain.asp
find more here