Hi,
just to contribute my 2 cents.

I recall having some similar problems with a nvme disk and debian/devuan install
some time ago, install apparently succeeded but system was unbootable and
it was solved by switching from legacy bios to uefi bios mode.

Chatgpt hints:

While not strictly mandatory in all cases,
using a UEFI BIOS is highly recommended and often necessary for booting Debian from an
NVMe SSD to ensure optimal performance, reliability, and proper detection.
Legacy BIOS often lacks native support for NVMe drives, meaning the drive may be visible
within the installer but fail to boot after installation.
Here is a breakdown of the requirements and best practices for installing Debian on NVMe:

Why UEFI is Recommended for NVMe

    Native Support: UEFI natively recognizes NVMe drives, while older Legacy/CSM BIOS may not.
    GPT Partitioning: UEFI requires the GUID Partition Table (GPT) format, which is essential
     for drives larger than 2TB and better for modern storage architectures.

Requirements for Successful Installation

    BIOS/UEFI Settings:
        Set the BIOS mode to "Pure" UEFI (disable CSM/Legacy mode).

    Ensure SATA Operation is set to AHCI

     Disable Secure Boot if you encounter issues with signed bootloaders, 
     although Debian 10+ does support it.

Installer Preparation:

    Ensure the USB installation media is created in GPT mode (e.g., using Rufus with "GPT" partition scheme).
    Boot the USB installer in UEFI mode.

Partitioning:

    During installation, ensure a dedicated EFI System Partition (ESP) is created on the NVMe drive 
   (typically 100-500MB, formatted as FAT32). 

Legacy System Workaround: 
     If your motherboard absolutely lacks UEFI, you might be able to use a third-party
     bootloader like Clover on a USB stick to boot to the NVMe drive. 

Untested and unchecked for AI hallucinations.

Ciao,
Tito