Getting circular dependency warning upon formulating username creation

So I am trying to create a sheet to help our HR Department create the emails for new hires. One of the issues is we use a format of First Initial Last Name as our naming scheme, but if you don't check it can double up with common last names. HR usually does not check for previous emails that currently exist.

Basic recreation I am trying to do is this:

Username: IFS(F2<>"", F2, IF(COUNTIF(A:A, D) > 1, E2, D2)

First Choice: LEFT(B2, 1) & B3

Second Choice: B2 & B3

What I want for A2: So basically if Override is set, i want it to use that. If no override is set, i want to check and see if First Choice is already found in column A, if it is already used then use Second Choice. I keep getting a circular dependency. I even tried having the calculation done in Column G, which works. But once I try and set A2 to G2, it gives the circular dependency error again.