Factory restoring my custom desktop PC. How can I ensure user name?

So I built a PC last year and I have recently updated to a new email address which of course messed up my Microsoft one drive link and other things. I decided that I am going to restore windows 10 due to a few reasons. I have previously uninstalled windows store app and my user folder name in the C drive is terrible due to my previous email.

I have a few questions!

1. Will restoring Windows 10 mess up anything on a custom built pc? Will I have to re-install bios or the like?

2. How can I ensure my user name for the user folder in C is the name I want it to be instead of taking letters from my gmail address?

1. No.

2. Create a local account when you set up the PC. See step #22 here Clean Install Windows 10 - Windows 10 Installation Upgrade Tutorials

Once you have signed in convert your local account to MS account (use Manage Your Account in Settings) and C:\Users\YourName will stay the same.

Thanks! :) I'll try it.

Best of luck :)

Will I be missing out on anything if I start with a local account and then change it to my Microsoft account?

Nothing at all. I always do it for same reason as you - I don't like the random name they make up.