OS corrupted + clean installation woes

Hello. Before I get started, I'd like to post a disclaimer that I'm pretty much a novice when it comes to installing Windows on my own, so please be patient with my cluelessness. I'm also currently posting from my really cheap backup PC so I won't be able to give you much system-specific info about my main PC that I'd need to actively look up within it.

I'm normally on W10 Home 21H2. The PC is also store-bought, bought it in a rush after my previous PC got wet & crapped out. So I was not involved in its installation process. I also feel it's important to tell you the whole story, in case something in it makes more sense to you than it did for me.

My problems started on Friday night. I used my PC for a couple hours before bed, no issues. Shut down normally, went to bed. Saturday morning (yesterday) I turned it on to continue my work...and nothing. Boots straight into the "keyboard region layout selection" screen, then it gives me 2 options - Turn Off PC or Troubleshoot/Advanced.

Repair Startup - didn't work any time.
System Restore - tells me to reboot & select an OS to restore = vicious loop.
Restore from image - didn't have one. Feel free to chastise me for that. MS Support told me the OS seems too corrupted and it wouldn't have worked anyway.
Command Prompt - didn't know what to do with it other than open Notepad.exe and use its basic file explorer to backup my main User folder & Program files to an external drive.
Change UEFI settings - pretty much to go into BIOS, not sure for what.
Uninstall updates (quality or feature) - I didn't want to touch it because the year before I had constant BSOD issues when I installed a new external backup drive that were fixed after I updated to newer feature update.

I have no idea what caused the corruption. I didn't make any changes to my PC that night. Just worked on my music for a couple hours and that's it. The only change that week was getting a new Logitech G502 Hero gaming mouse earlier in the week, because my old trusty MX518 finally kicked the bucket, and I installed the Logitech Hub - only changes I made were mouse-exclusive cosmetic, tinkered with the light colors.

Via CMD I tried to perform a disk check (sfc and chkdsk and whatnot), but it kept showing a X:\ drive instead of C:\ and there was nothing I could do to change that. Couldn't even boot safe mode. MS Support said the OS is too corrupted.

So after spending 3 hours chatting to MS Support and trying to figure out any way I could perform an in-place installation so I wouldn't have to reinstall all of my programs from scratch, they told me my only option is to do a clean installation.

I hopped onto this bare-bones PC for the first time in 2 years, created a new 21H2 installation flash, and went in. And then I reached this problematic area. My googling led me to these threads below:

Delete or format partition during clean install windows

Cant install Windows 10 because of GPT partition error message

All of my drives were showing the error "can't install Windows because of GPT partition style". Googling yielded a ton of info, which made my mind spin. The only options I could see was Delete and Format. MS Support told me to use the CMD "diskpart" commands to "clean" the drive and convert to MBR.

But googling told me it's better to switch to UEFI, that way I can install as GPT without issues and I'd get the touted performance-improvement benefits etc. My default booting type in the BIOS from what I remember is "UEFI+legacy".

Here's where I don't quite know how to proceed. I'm the paranoid type who is normally afraid to touch anything in case it gets corrupted.

Off the top of my head, my best option would be to boot the installation flash drive via UEFI so that it would let me install GPT style - which according to MS Support would create another partition on the primary drive with the corrupted OS, install a fresh OS and create a Windows.old folder with all the other files on it.

Or format to MBR and lose all the files (I've tried to backup most of them but you never know #paranoid).

I'm also wary of messing with the default BIOS settings, so I'm also wondering what kind of effects it would have switching to UEFI boot exclusively from here on out. Googling didn't provide any helpful info.

And one last question: I have two 8tb external backup drives. Should I leave them plugged in during the fresh installation or should I disconnect them? MS Support says "leave them plugged in", Googling says "lose them".

I ask because I'm slightly worried that the old BSOD issues will start again once I install them on the fresh OS if they're not already plugged in during installation. MS Support says it's better to keep them plugged in so that the OS installation detects them & installs/downloads the best drivers for them from the get go.

Sorry for this wall of text, but I'm tech-paranoid and I like to ask as many questions as I can. Only way to learn. That and personal experience. Thanks in advance!

@Ghot

many thanks for your speedy reply, though you've underestimated my noviceness and I'd appreciate it if you could provide a more detailed response.

1. I can unhook the external ones but I'm afraid to touch the internal ones - one 4tb HDD and one 1tb Samsung SSD for my gaming library. OS drive is a 256gb SSD. Why is it important that I unhook them?

2. There's no option to do so. Thinking back, my build might've been 21H1 (because I'm always annoyed by feature updates resetting my GUI changes). And I haven't found a way to create an installation flash drive with 21H1, it always defaults to the latest W10 build.

3. Use the installation media to install GPT, as in? To boot it as strictly UEFI, and...use the Delete option during the installation window?

@Ghot had to dig through my boxes, and thankfully I found the old manual.

My motherboard is MSI B85M-E45. And I discovered some of the rest on the sticker on the box.

i7-4790 3.6GHz/4.0GHz 4 cores 8mb cache
GTX960 4gb GDDR5
16gb DDR3

"You will only be able to do a repair install of Windows 10 from within Windows 10. You will not be able to do a repair install at boot or in Safe Mode."

In-place upgrade is impossible because I can't boot to desktop. I can't boot anything. Always takes me to the Troubleshoot/Advanced or Turn off PC screen upon system start.

@Ghot the default boot mode is UEFI+legacy in my BIOS. so I wouldn't need to boot the flash drive as strictly UEFI? I can just delete the existing partition during this screen and it won't give me the GPT error later?



Do I also have to delete every partition on Drive 0 (recovery, system, MSR) or just the "primary"?

@Ghot oh so it's giving me the GPT error because the SSD is loading as UEFI but the flash drive isn't, right???