Update: I heard back from DigitalOcean, and they're going to extend my credits through the end of the year.

"We would like to extend your referral credits referenced in the previous email through December 31, 2021. This specifically includes those credits which were set to expire on May 25th due to lack of use. Any new credits earned through the referral program will be subject to the 12 month expiration policy."

I don't know if they're going to do this automatically, so you might want to reply to their email.

This isn't perfect, but the rest of the year gives me more flexibility than one week. Hopefully I can use them up.