Need help for rooting Blackview new BV9900E - Android Q&A, Help & Troubleshooting

Hello,
I received a BV9900E a few days ago. it's a nice Rugged phone.
But... I would like to put a custom launcher with custom widgets and custom animated wallpaper. But if I shut down the phone, I lost all my settings, the widgets and wallpaper. All must be reinstalled, and/or reconfigured.
I try to use whiteliste, did several tries and try various solutions found on the net but nothing helped ... I lose everything on each restart. Only stock widgets work.
I think I have found the beginning of an explanation here: https://bbs.blackview.hk/viewtopic.php?t=532725
But for edit Build.prop, i have to root the phone.
I am not an expert for rooting phone and try like on the Blackview topic, but i don't find BV9900E/Android 10 specific stock ROM for now.
I only can find the BV9900E_S1_200929 rom and not the BV9900E_EEA_S900AA.
I try to root with the BV9900E_S1_200929 rom, Adb, magisk, but nothing happend.
Any solution is welcome !
Thank you in advance for your answers.
Modèle : BV9900E
System : Android 10
Num serie : BV9900EQEE002138
Build : BV9900E_EEA_S900AA_V1.0_20201103V04

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Rooting a phone's Android doesn't require to re-flash phone's Stock ROM. A phone's Android is rooted when su binary got installed on it.
To prepare rooting you have to
Unlock phone's bootloader
Disable Android's DM-Verity & AVB locks
Flash a Custom Recovery like TWRP what is at 100% suitable to phone
To root Android you have to
Flash by means of this Custom Recovery SuperSU.zip or similar

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Ok, thank's jwoegerbauer...
Unlock bootloader is ok. but impossible To disable DM-Verity and there is no TWRP for the BV9900E yet...
And i don't know, it's very strange because i can't mount system when i am on recovery mode for example.
Blackview has only one answer: It's not possible to do that.
I am not an expert with the handling of phones but I am not quite bad with technology either. I already had 2 phones that I ended up rooting but there, I do not succeed. I may have to wait a bit because this phone is brand new, the solutions may come from someone more expert than me.

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Disabling Android's SElinux, DM-Verity & AVB is achieved by running specific ADB / Fastboot commands.
Mouning /system RW requires Android is rooted
Compile TWRP at your own if none matching available.

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Ok, that sounds simple to you.
But when I search the internet there are thousands of answers that go in all directions ... It is not so easy for the uninitiated like me.
Do you have some links for illustrate what you say?

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To disable the mentioned 3 locks you may consider to run a Windows CMD script like this draft
Code:
@echo off & setlocal ENABLEDELAYEDEXPANSION
pushd "%~dp0"
set "adb=C:\ADB-FASTBOOT-R30\adb.exe"
set "fastboot=C:\ADB-FASTBOOT-R30\fastboot.exe"
set "su=C:\SU-Binary\su"
set "tmpdir=%TEMP%\%RANDOM%%RANDOM%"
set "su_binary_location=/data/local/superuser"
set "su_tmp=!su_binary_location!/su"
set /a cnt=0"
mkdir "!tmpdir!" 2>nul
if NOT exist "!tmpdir!\" ( goto :end )
set "dev=" & set "name="
!adb! devices
!adb! shell stop
call :install_temporary_root
!adb! shell "mkdir -p -m0666 /data/local/vbmeta"
for /f "tokens=1,4 delims= " %%a in ('!adb shell "!su_tmp! -c 'cat proc/mtd'"') do (
set "dev=%%a" & set "name=%%b"
echo !name! | findstr /C:"vbmeta" >nul
if !errorlevel! EQU 0 (
set "dev=!dev::=!""
!adb! shell "!su_tmp! -c 'dd if=/dev/mtd/!dev! of=/data/local/vbmeta/!name! bs=4096'"
!adb! pull /data/local/vbmeta/!name! "!tmpdir!"
set /a cnt+=1
)
)
!adb! shell "rm -Rf /data/local/vbmeta"
if !cnt! EQU 0 ( goto :end )
!adb! shell "!su_tmp! -c 'setenforce 0'"
!adb! reboot fastboot
!fastboot! devices
for /f "tokens=*" %%a in ('dir /B "!tmpdir!"') do (
set "vbmeta_img=%%a"
if NOT [!vbmeta_img!]==[] (
set "vbmeta_partition=!vbmeta_img:.img=!"
!fastboot! --disable-verity --disable-verification flash !vbmeta_partition! "!tmpdir!\!vbmeta_img!"
)
)
!fastboot! reboot
:end
rmdir /S /Q "!tmpdir!" 2>nul
!adb! shell start
popd
endlocal & exit
rem
:install_temporary_root
!adb! shell "mkdir -p -m0755 !su_binary_location!" 2>nul >nul
!adb! push !su! !su_binary_location! > nul
!adb! shell "chmod 0777 !su_tmp!"
!adb! shell "chown 0.2000 !su_tmp!"
goto :EOF
where you of course have to adjust the pathname of ADB & Fastboot executables and the SU-binary
FYI: Have not tested such a script, it's on you to fix bugs if any given.

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With regards to compile a TWRP at your own:
look inside here.

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I also bought a bv9900e and have managed to unlock the bootloader, as guided here : LINK
Can i get some help with compiling TWRP ?