Help with dual booting Win 10 and Ubuntu

But did I do the right things to switch my set up to be the same like what you were suggesting? If I boot into the Win 10 disk then I see the nice Win 10 dual boot menu. If I select Ubuntu then yes I do see the GRUB2 menu but I guess I could play around with the grub config settings in linux so that menu doesn't show? Or was there something else I did wrong?

No, You did right. By default GRUB always shows the Boot Menu. I forgot to mention that you need to configure GRUB so that it will hide the Menu.
You can follow this link to hide it: How to Hide Grub Menu in Boot of your Linux Machine
or in Linux, you can install Grub Customizer and set visible to NO and Timeout=0.

Why do you really not like GRUB2 being the main bootloader? Is it because in your case you want to use Windows 10 most of the time so it doesn't make much sense?

Never said I did like or did not like. As a Windows user most of the time, Windows Manager is easier to maintain and fix or rebuild. Not just Windows users but also lots of Linux users complain about it. There's a lots of questions in this forum that users having problems getting stuck with Grub Loader.
I install Linux on both Legacy and UEFI. With UEFI, if you have Windows OS in the PC then Linux will take over control of Windows Boot Manager also and leave its own EFI System partition empty so I have to move it back to where it belong, still when you access the boot menu, there's 3 different entries for Linux to boot from, why would they do that ? I don't know but it's messy, the 3 entries are: /EFI/Boot/Bootx64.efi, /EFI/ubuntu/grubx64.efi and /EFI/ubuntu/shimx64.efi while only grubx64.efi is needed. I had to manually rename the other 2's so only one will show up in the boot menu.

In my use-case I would prefer to use Ubuntu 90% of the time. From my research I learnt that GRUB2 can boot Windows and Ubuntu of course without needing to do an extra boot and you also said yourself that Windows bootmgr has to do an extra boot to make Ubuntu boot. I think that is inefficient and will cause more wear and tear on my hardware.

Cannot tell you what wear and tear little thing like that would do anything to your hardware. I have a 15 year old PC and despite of my abuse, it's still running like new.

I'm reading your interesting post in this thread but I don't understand your first point 2. and point 4. From my testing the GRUB boot loader does not need to be in its own partition so Ubuntu can can boot stand alone. It booted just fine for me with GRUB in /dev/sda or in /dev/sda1. Well each time I get the GRUB menu but I don't think the location of GRUB determines if you get the menu or not but is determined by how you edit the GRUB config files. No?

Well, I have to repeat again. I just want everything in one place: Grub Loader+Linux OS so if I want to remove it then it's gone, nothing related to Linux remained in the PC.

For your point 4: "Any update from Windows won't affect LINUX and vice versa." What kind of updates do you mean that might cause problems and what kind of problems? In my first set up with GRUB in /dev/sda and being the main bootloader it wasn't affecting the Windows installation at all because they are both on separate disks just GRUB has a pointer to Windows. Windows doesn't know that Ubuntu exists... I don't think? The only thing is if I restart the computer then it will default boot into Linux and maybe that will prevent a Windows update from completing that required a reboot. But this is hardly a show stopper? This problem is unavoidable in a dual boot set up where Windows is not the boot default.

For MBR, besides putting GRUB in the MBR, it also modifies the Windows Boot Manager in order to take whole control, as the case for UEFI, It moves everything from its own EFI System Partition to the Windows EFI System Partition, I then took a look at one file Bootx64.efi and compare one before install Linux and one after Install Linux, the difference in size is about 200K, with a program (Beyond Compare) to compare the difference between the 2's and they are big difference.At the end, I still think you should consider using the boot menu as advised by @Megahertz and I said it before, this is how I use it also.

1. Turn off the Grub boot menu. set it to first boot since you work with Linux most of the time.
   This way you don't even have to wait 10 seconds count down before Grub boot up.
2. Boot Windows when needed via boot menu.