Just some advice: I would start a dispute with your credit card issuer.
The DoT may eventually rule in your favor, but the key word is "eventually". By the time they get around to your case, the flight credit will almost certainly have expired.
I had a pandemic-era dispute with an airline (the airline refused refund and only offered credit, when they cancelled the flight, which is against DoT rules requiring a refund for canceled flights). I filed a DoT complaint and also initiated a chargeback with my CC (documenting the DoT rules clearly for the CC company). After a couple rounds of back and forth with Chase/Visa, they sided with me and charged back the full amount. At that point I promptly forgot about the DoT until about a year later, I got a really nice letter and phone call from someone at DoT indicating they had found my complaint valid. Of course by then it was moot.
TLDR: the DoT enforcement was fair and polite, but took forever and if you actually want your money back your best option is the credit card company.