I’m putting my numbers into a spreadsheet trying to determine the most housing I can afford.
The trick to spreadsheets is realizing that your life isn't a predictable spreadsheet. Make sure you've gone a couple of months living according to the numbers in your spreadsheet so that you know they're accurate.
Is it a bad idea to only consider what I have left each month after EVERYTHING has been paid (mortgage, taxes, utilities, HOA, food, gas, insurance, maxing 401k and Roth IRA, random supplies, etc. (and no other debt and already have 6 months emergency funds))?
First, thank you for properly nesting your parentheses.
Second, make sure you don't leave out sinking funds. Eventually, you'll need a new(er) car, for instance.
As a homeowner, you'll also need sinking funds for semi-predictable expenses like a new roof and new AC. All the 5-digit expenses that aren't someone else's problem anymore. And extra emergency funds for the non-predictable expenses, like "the sewer connection broke and leaked into the basement and the whole lawn needs to be dug up so it can be replaced". And extra income to replenish all these funds after they're used.
Failure to account for this will go through a few phases each time something happens: