If you're going to start playing around with multiple OS's. Is really in your best interest to start using a data partition. Or better yet, start keeping your files somewhere else entirely and then backing that place up...

You can't really brick a PC by doing anything to the boot drive though. You may screw up the OS, but the bios is there to make sure you can boot off something else if need be...

I am unsure of what or how many files you need but ever think about using something like a flash drive to save your files to it and then wiping drive? I know sometimes you get so caught up with complex problems that you forget about the simple solutions.

You know one of the easiest and safest ways of switching base os? Replace the ssd (or m.2).

They're ridiculously cheap now and, after copying the installer files to a usb stick, unplug your old ssd and plug in the new one. Then you can go back fully if it doesn't work out.

I recommend using Transwiz to zip up your user profile, you can move the .trans.zip file to a neutral location (external drive, network storage, etc). Of course if you have valuable information stored outside the C:\Users folder, back it up as well. Now you should have a system you can safely mangle, destroy and rebuild without worrying about user data. Once you've built your new setup, extract the zip folder into /home/[your_name] or ~ and you're all set.

Don't turn on overclocking or do anything that pops up a warning and you should be okay doing pretty much anything to the bios. Yes, you can make you machine fail to start (fail to post) but it's a reset jumper or button away from being back to default settings again.

Make absolutely sure that you're not touching the ubuntu partition while getting rid of windows.

A backup of at least your home directory is recommended.

Then just remove the windows partition from ubuntu and install garuda to it as usual. You'll probably need to use grub-mkconfig after installation to make sure grub shows both ubuntu and garuda. The arch wiki has a good guide on that.