Or, at least I want to slow down with expansion. 

Looking to start slow and expand up!

As of early 2024, inventory levels were about 3.79% higher than the five-year quarterly average, indicating a slow but steady increase in supply​​.Demand and Buyer BehaviorThe Birmingham market will continue to attract out-of-state buyers, with significant interest from cities like Chicago.

I usually just send a screenshot of a bank accounts, but I have seen others get their bank to write an official dated letter saying they have at least X available to withdraw immediately.I would not consider the letter you attached as a proof of funds, nor a contract offer, its more like a letter of intent? 

What are everybody's thoughts on using a 401k loan or withdrawal to fund the downpayment?

It gives you a false feeling of success, but really slows you down.

Why do you think Pigeon Forge and Galinburg is so slow right now?

Also, what's your plan if the market slows down or stagnates?

Slow and steady!

I see a slow slog downward until we hit the balance point mostly because the rates will remain elevated 6%+ for another year at least.