I understand what you are saying. Itās possible the offline install didnāt have the line in default grub command line. That doesnāt matter. It just means you need to add it if it isnāt there. os-prober does have to be installed if it isnāt installed either. I would have thought it would have been? But you can check and if it isnāt install it.
Check:
pacman -Qi os-prober
If it isnāt installed then install it. Then run it and update grub command again. Iām not sure where your at? As far as turning off secure boot, I only sent you those videos as a reference. The delete platform key (PK) i mean is fine but thatās referencing Windows. It should be just a matter of setting it in UEFI when you clear the keys and save and exit Bios screen.
Woah so many postsā¦ @ricklinux kudos for the troubleshoot and patience, hopefully this is solved.
Well i know itās a lot of posts. Itās a brand new computer and installing Windows on it and then EOS is easy if everything falls into place. This issue with secure boot on this board has been a problem as it wonāt pick up windows and add it to grub. We did have success but the OP wanted to reinstall KDE after getting it resolved and itās either doing the same or itās a matter of os-prober wasnāt installed. Iām not sure i just see this now.
The os prober thing is tricky to catch, itās a small hidden thing, and difficult to find in the wiki until you found it.
Edits: I am working on a dual boot now installed xfce yesterday. There was no os-prober installed and the GRUB_DISABLE_OS_PROBER=false was not set to false out of the box. Just FYI. Fixing that to show windows in my grub menu.
Package os-prober will be installed by default with the next ISO
(the ISO will be released quite soon).
This is only on the second time the OP is installing. The first time the issue was that it would not write or read the file in order to write it because secure boot wasnāt disabled. This time Iām not sure because os-prober may not have been installed? Iām waiting to see where the OP is at with it.
Thanks 
@Elnath
Just wondering if you checked? os-prober may not have been installed so that would be an issue.
Success
sudo pacman -S os-prober
sudo grub-mkconfig -o /boot/grub/grub.cfg
[elnath@elnath-ryzen ~]$ sudo grub-mkconfig -o /boot/grub/grub.cfg
Generating grub configuration file ...
Found theme: /boot/grub/themes/EndeavourOS/theme.txt
Found linux image: /boot/vmlinuz-linux
Found initrd image: /boot/amd-ucode.img /boot/initramfs-linux.img
Found fallback initrd image(s) in /boot: amd-ucode.img initramfs-linux-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Windows Boot Manager on /dev/nvme0n1p1@/efi/Microsoft/Boot/bootmgfw.efi
Adding boot menu entry for UEFI Firmware Settings ...
done
[elnath@elnath-ryzen ~]$
Did the job. Finally. 
We did have success but the OP wanted to reinstall KDE after getting it resolved and itās either doing the same or itās a matter of os-prober wasnāt installed. Iām not sure i just see this now.
Doing offline (XFCE) install has os-prober installed thats why there grub could add windows boot manager wheras KDE has no os-prober installed by default.
I did not know os-prober was an package I just assumed its setting name in grub.
What we failed
The first time the issue was that it would not write or read the file in order to write it because secure boot wasnāt disabled.
Honestly, I still dont know how this secure boot thing works. I could NOT target windows efi partition still to this day with /boot/efi I had to create MY OWN efi partition in gparted.
This still is fine, I just hate when things donāt work the way they should and want to fix the issue if thats the case.
We did accomplish many things but having windows efi partion to have both
- Windows
- Linux
entry was not done.
Well, whatever I already invested much time into this dont want drag it on any longer when the main goal was accomplished. 
@ricklinux Thanks for taking your time to help me! I was suprised you stick to very end with me. But holy shit I was at some parts very annoyed. Error after error and stuff. 
I think that the secure boot on this motherboard works and gets enable when you add secure keys so when you remove them it gets disabled. Iām sorry for all the confusion as os-prober and grub stuff got changed by upstream and i do a lot of testing on development ISOās Iām hardly ever using the currently released ISO. I forget about a lot of things because it gets changed and Iām not using it anymore.
As far as the Windows efi partition goes it really doesnāt matter that you have an efi partition on eos. Itās not like your wasting a bunch of space. Itās pretty small as Windows efi is only 100 MB. Knowing how your hardware works is more important and it is working to get it installed. I wouldnāt be too hung up on it. Itās just one of those hardware issues. I do personally think you should at some point update your UEFI Bios to the very latest version. It is very easy to do. The best way is to use the ASUS EZ flash 3 Utility which is in your UEFI Bios screen.
I think this is the correct motherboard manual pdf. You have an Asus Prime B550 Plus right?
E16536_PRIME_B550-PLUS_UM_WEB_051420.pdf (2.4 MB)
Instructions are in section 2.3
As i said before you download the new updated UEFI Bios files. They are usually in a compressed file. You extract them and the extracted files are usually in a folder. You just copy the folder with the files to a usb. Then you boot into the flash utility in the Bios setup. It does say to set the Bios to defaults but that is only necessary if youāve made a number of changes. When i do a flash i always go into the UEFI settings and check all of them after and set them accordingly because usually the new UEFI Bios will be set to defaults.
Iām just providing you with this info here so if you so decide at some point you want to update it then look over the instructions in the manual. Hopefully i have the right one. I donāt know if you have the motherboard manual with your system? But everything can be found on the Asus website under your particular motherboard. You may even find that the new update changes the UEFI Bios settings and it may be slightly different or it could be the same. Anyway itās your choice what you do. But itās all about learning and understanding your own hardware. Thatās half the battle. Iāve been doing this kind of stuff for a long time. I may not have an answer for everything and Iām not a know it all. I only know what i know and i learn what i donāt know and i donāt give up easily. I can get frustrated too. 
Edit: You could close this thread also as i think we have finally landed on EOS!
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