Passcode use

You can definitely set up an alpha/number passcode but you will have to check about the specifics of starting with a uppercase letter. To set this up, open Settings and select Face ID and Passcode. Then scroll down to Change Passcode. Enter your current passcode then when prompted to enter the new passcode, select Passcode Options. Custom Alphanumeric is one of the options.

I can confirm that it works. I just changed my passcode to a custom alphanumeric code. The uppercase or lowercase letter(s) can go where ever you want. Interesting. When you are prompted for the passcode, you get the full keyboard.

The Passcode only uses numbers, no letters or symbols.

Check Passcode options on your phone. Appears to be possible to enter an upper case letter first. Whether it will be accepted is not confirmed.

Thank you for correcting if I was wrong!