I was also able to find that driver and posted it in another thread. I can fully believe that it wasn't available when OP searched for it, but maybe they just didn't find it due to Dell's support site being somewhat difficult to navigate for people who aren't used to troubleshooting Dell computers and especially clean installing or dual booting them.
One workaround is to install in AHCI mode and then use the safe mode method to switch the storage mode out from under Windows, install missing drivers for the desired boot mode under safe mode if they aren't installed automatically by Windows, then reboot to exit safe mode.
However, OP doesn't seem to have the option to switch to anything other than RST, which could be due to BIOS settings and/or Windows settings, as well as flags on your partitions that cause Windows to boot in read only mode. You might need to toggle some settings for legacy/uefi compatibility mode, reset BIOS/EFI to (un optimized for Windows 8/10/11) default settings, disable secure boot temporarily, etc to let the EFI allow you to toggle the boot mode on some Dell BIOSes, however, especially in OP's case where they appear to not have any alternate drive modes besides RST in BIOS/EFI. In especially bad cases, you might even need to update the BIOS to enable these modes and/or wipe the EFI partitions and possibly the entire boot drive, as sometimes the BIOS/EFI is inserting itself in the boot process via "dirty bits" and/or Fast Startup mode.
One workaround is to install in AHCI mode and then use the safe mode method to switch the storage mode out from under Windows, install missing drivers for the desired boot mode under safe mode if they aren't installed automatically by Windows, then reboot to exit safe mode.
https://www.tenforums.com/tutorials/22631-enable-ahci-window...
https://www.tenforums.com/drivers-hardware/15006-attn-ssd-ow...
However, OP doesn't seem to have the option to switch to anything other than RST, which could be due to BIOS settings and/or Windows settings, as well as flags on your partitions that cause Windows to boot in read only mode. You might need to toggle some settings for legacy/uefi compatibility mode, reset BIOS/EFI to (un optimized for Windows 8/10/11) default settings, disable secure boot temporarily, etc to let the EFI allow you to toggle the boot mode on some Dell BIOSes, however, especially in OP's case where they appear to not have any alternate drive modes besides RST in BIOS/EFI. In especially bad cases, you might even need to update the BIOS to enable these modes and/or wipe the EFI partitions and possibly the entire boot drive, as sometimes the BIOS/EFI is inserting itself in the boot process via "dirty bits" and/or Fast Startup mode.
https://superuser.com/questions/1554458/how-does-windows-loc...