[OnePlus 7T Pro (Regular & McLaren) ROMs & Dev.] MesSYou:
For all, who wants to choose the Slot where DerpFest should be installed. A | B | Both.

Here's my script which i've been used to update/install.

Just put it in a textfile and rename it to ".bat"

Lean back and get more s**tloads of coffee until it's done.

@NurKeinNeid - Thank you very much, for giving the OTA Updater back!

Best regards!

@echo off
echo "Script powered by MesS_U"
echo "FastbootD - DerpFest Slot installer"

fastboot devices
pause

:choice
set /P c="Do you want to wipe Data partition? [W]ipe | [S]kip >"
if /I "%c%" EQU "W" goto :Wipe
if /I "%c%" EQU "S" goto :Skip

:Wipe
fastboot -w

:Skip

:choice
set /P c="Which Slot do you want to Flash/Update? Slot[A] | Slot[B] | Both[X]? >"
if /I "%c%" EQU "A" goto :SlotA
if /I "%c%" EQU "B" goto :SlotB
if /I "%c%" EQU "X" goto :Both

:SlotA
echo "Processing Flash/Update Slot A"

fastboot flash system_a system.img
fastboot flash system_ext_a system_ext.img
fastboot flash boot_a boot.img
fastboot flash odm_a odm.img
fastboot flash product_a product.img
fastboot flash vbmeta_a vbmeta.img
fastboot flash vbmeta_system_a vbmeta_system.img
fastboot flash vendor_a vendor.img
fastboot flash dtbo_a dtbo.img

echo "Flash Slot A Completed"

:choice
set /P c="Reboot device [Y]es | [N]o | [R]ecovery ? >"
if /I "%c%" EQU "Y" goto :AYes
if /I "%c%" EQU "N" goto :ANo
if /I "%c%" EQU "R" goto :ARecovery

:AYes
fastboot reboot
pause
exit

:ANo
echo "Reboot aborted"
echo "Done! Please Exit"
pause
exit

:ARecovery
fastboot reboot recovery
pause
exit

:SlotB
echo "Processing Flash/Update Slot B"

fastboot flash system_b system.img
fastboot flash system_ext_b system_ext.img
fastboot flash boot_b boot.img
fastboot flash odm_b odm.img
fastboot flash product_b product.img
fastboot flash vbmeta_b vbmeta.img
fastboot flash vbmeta_system_b vbmeta_system.img
fastboot flash vendor_b vendor.img
fastboot flash dtbo_b dtbo.img

echo "Flash/Update Slot B completed"

:choice
set /P c="Reboot device [Y]es | [N]o | [R]ecovery ? >"
if /I "%c%" EQU "Y" goto :BYes
if /I "%c%" EQU "N" goto :BNo
if /I "%c%" EQU "R" goto :BRecovery

:BYes
fastboot reboot
pause
exit

:BNo
echo "Reboot aborted"
echo "Done! Please Exit"
pause
exit

:BRecovery
fastboot reboot recovery
pause
exit

:Both
echo "Processing Flash/Update 1/2 - Slot A"

fastboot flash system_a system.img
fastboot flash system_ext_a system_ext.img
fastboot flash boot_a boot.img
fastboot flash odm_a odm.img
fastboot flash product_a product.img
fastboot flash vbmeta_a vbmeta.img
fastboot flash vbmeta_system_a vbmeta_system.img
fastboot flash vendor_a vendor.img
fastboot flash dtbo_a dtbo.img

echo "Flash/Update 1/2 successful"

echo "Processing Flash/Update 2/2 - Slot B"

fastboot flash system_b system.img
fastboot flash system_ext_b system_ext.img
fastboot flash boot_b boot.img
fastboot flash odm_b odm.img
fastboot flash product_b product.img
fastboot flash vbmeta_b vbmeta.img
fastboot flash vbmeta_system_b vbmeta_system.img
fastboot flash vendor_b vendor.img
fastboot flash dtbo_b dtbo.img

echo "Flash/Update 2/2 successful"

echo "Flash Slots A|B completed"

:choice
set /P c="Reboot device [Y]es | [N]o | [R]ecovery ? >"
if /I "%c%" EQU "Y" goto :XYes
if /I "%c%" EQU "N" goto :XNo
if /I "%c%" EQU "R" goto :XRecovery

:XYes
fastboot reboot
pause
exit

:XNo
echo "Reboot aborted"
echo "Done! Please Exit"
pause
exit

:XRecovery
fastboot reboot recovery
pause
exit