I downgraded Ubuntu to 32 bits, now have problems reinstalling it

I've had a fully functional Ubuntu 20.04 environment, until I recently installed the HTK library using htk-tools_3.4.1-ubuntu14_i386.deb. In the process, everything 64 bits was removed for now obvious reasons and the system was essentially downgraded to 32 bits. Virtually nothing worked. Realizing my mistake, I quickly reinstalled Ubuntu. The installation itself works and is fine, except maybe that it now performs a file system check on every start-up, which it didn't do before. But... I get problems when I install NVIDIA drivers (and I will also need CUDA). I've installed the drivers multiple times (Software center -> Additional drivers, sudo apt install...), both -470 and -495 versions. The driver installation goes without complaints, and right afterward nvidia-smi works correctly.

However, when I reboot the computer to take the drivers into effect, something goes wrong. When rebooted, I no longer have access to ethernet (no ethernet options in Settings) and some apps are also gone (same that disappeared in the 32 bit downgrading process). Removing the NVIDIA driver and enabling Nouveau doesn't bring ethernet back. I have reinstalled Ubuntu from scratch multiple times, but it's always the same after rebooting after the NVIDIA driver installation.

I currently suspect that the 32 bit excursion made some modification to GRUB, and that the NVIDIA driver installation somehow activates it, and boots in a 32 bit like state.
Now I have a working Ubuntu installation (without NVIDIA), and I have installed the GRUB repair tool and ran an analysis, but this is a bit above my head. I've pasted the log below, if it helps.

Would it be advisable to let this tool repair GRUB (which options should I use) or repair it some other way? Or do I need to do something more drastic, such as deleting the EFI partition or the whole partition table altogether?
Or is there another, more likely, problem causing these issues?

My drives are:
nvme0n1 with 3 partitions: 1) FAT32/EFI, 2) swap and 3) etx4 for Ubuntu
nvme1n1 with 1 partition: ext4 for /home
(sdx HDDs for Windows installation and data storage)

I don't necessarily need Windows, so I would be ok if the bootloader was removed, if necessary. Ubuntu is the primary OS, and I do not want to change anything in BIOS, which the repair tools seems to suggest.

Below is the log from the boot repair tool:

boot-repair-4ppa161                                              

============================== Boot Info Summary ===============================

 => Grub2 (v1.99-2.00) is installed in the MBR of /dev/nvme0n1 and looks at 
    sector 671999480 of the same hard drive for core.img, but core.img can not 
    be found at this location.
 => No boot loader is installed in the MBR of /dev/nvme1n1.
 => No boot loader is installed in the MBR of /dev/sda.
 => No boot loader is installed in the MBR of /dev/sdb.

nvme0n1p1: _____________________________________________________________________

    File system:       vfat
    Boot sector type:  FAT32
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  
    Boot files:        /efi/BOOT/fbx64.efi /efi/BOOT/mmx64.efi 
                       /efi/ubuntu/grubx64.efi /efi/ubuntu/mmx64.efi 
                       /efi/ubuntu/shimx64.efi /efi/ubuntu/grub.cfg

nvme0n1p2: _____________________________________________________________________

    File system:       swap
    Boot sector type:  -
    Boot sector info: 

nvme0n1p3: _____________________________________________________________________

    File system:       ext4
    Boot sector type:  -
    Boot sector info: 
    Operating System:  Ubuntu 20.04.3 LTS
    Boot files:        /boot/grub/grub.cfg /etc/fstab /etc/default/grub

nvme1n1p1: _____________________________________________________________________

    File system:       ext4
    Boot sector type:  -
    Boot sector info: 
    Operating System:  
    Boot files:        

sdb1: __________________________________________________________________________

    File system:       
    Boot sector type:  -
    Boot sector info: 

sdb2: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Windows 8/10/11/2012: NTFS
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  Windows 8 or 10
    Boot files:        /Windows/System32/winload.exe

================================ 2 OS detected =================================

OS#1:   The OS now in use - Ubuntu 20.04.3 LTS CurrentSession on nvme0n1p3
OS#2:   Windows 8 or 10 on sdb2

================================ Host/Hardware =================================

CPU architecture: 64-bit
Video: NVIDIACorporation from   NVIDIA Corporation
BOOT_IMAGE of the installed session in use:
/boot/vmlinuz-5.11.0-46-generic 
[...]

After what feels like a thousand re-installs and reboots, I seem to have NVIDIA driver, CUDA and cuDNN installed and working. Several lessons learned... Thank you all for your time and help!

I ended up wiping the GPT of my system & boot SSD, and physically disconnecting an HDD on which I have a Windows installation - and followed Nmath's advice to allow 3rd party proprietary software and download updates while installing. From there, installing the other packages went smoothly.

Again, thank you.