Dualboot: Ubuntu not booting at all. Windows booting goes through GRUB command line

Question

I installed Ubuntu in addition to my existing Windows 10 using a pendrive.But unable to boot into Ubuntu.
Initially (before try 4), only Windows Boot Manager and the hard disks were visible in BIOS boot order.
Now (after try 4), PC boots into GRUB command line and when exited from that, WBM and 2 copies of my primary hard disk are visible in the Boot Manager, but 1 of them returns me to GRUB Command Line, 1 of them boots Windows.

Partitions:
Installed Ubuntu on the same Hard disk as my Windows, with manual partitioning.
I have 2 hard disks - 1 main SSD and an external HDD.
/boot 512MB SSD; swap partition - 16GB SSD; / - 75GB SSD; /home - 95GB HDD2;
efi partition - 100MB SSD (same efi partition as WBR)
I have assigned an EFI System Partition in the efi partition containing Windows Boot Manager.
https://i.sstatic.net/C0fbG.png

What I tried:
From the installation media - SanDisk USB (I use Shift+Restart from Windows 10 to access the USB)

1. I tried using efibootmgr to adjust the boot order.
   efibootmgr mentions the Ubuntu installation.
   But everytime I reboot, the order remains as-is, except the Ubuntu's ID gets removed automatically from the order. It is still visible in the list though.
   Before try 4 - https://i.sstatic.net/8OaBq.png

2. Installing GRUB. After a lot of errors and debugging, this is what I finally got - that grub was installed.

   sudo mount /dev/sdb7 /mnt
   sudo mkdir /mnt/boot/efi
   sudo mount /dev/sdb5 /mnt/boot/efi
   sudo grub-install --efi-directory=/mnt/boot/efi --target=x86_64-efi /dev/sdb5
   grub-installed 

   Checking if GRUB is indeed installed.

   sudo file -s /dev/sda
   /dev/sda: DOS/MBR boot sector

   sudo dd bs=512 count=1 if=/dev/sda 2>/dev/null| strings
   ZRr=
   `|f
   |f1
   GRUB
   Geom
   Hard Disk
   Read
   Error
   

3. Everytime Windows boots, it says "Scanning and repairing a drive (S:)"
   and S:/ is where I had assigned the swap partition. I suspect Fast Startup is on.
   Disabling Windows Fast Startup and Hibernation - but these 2 options were not visible in my Control Panel > System Settings. So I turned off just hibernation from Windows terminal.

   powercfg /h off

4. When I ran
   

   bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

   and shut down and booted, I ended up at the GRUB Command Line, instead of the GRUB Menu.
   I typed exit, pressed Escape, etc - but nothing happened. I hard shut down the laptop.
   When I booted again, and typed exit this time, it showed me 2 options in the Boot Manager - both were my SSD hard disk.
   Boot Manager - https://i.sstatic.net/eVQ30.jpg
   Selecting the 1st one returned me to GRUB Command Line.
   Selecting the 2nd one booted Windows.
   efibootmgr -v output - https://i.sstatic.net/MxisE.png
   

Status + TLDR:
My laptop is an Acer Aspire E5-575G.
Boot Mode:UEFI.
Existing OS:Windows 10. New OS:Ubuntu 20.04 - not booting at all.
Secure Boot is on. GRUB installed. Hibernation off.
Booting lands me on the GRUB Command line.

Any solutions/suggestions? Thank you.

Answer 1

boot-repair's Recommended Repair did it.
I had been putting it off because of the risk I'd read about, but it worked cleanly.