Is there a way to combine multiple daily digest emails into one daily digest email?

Unfortunately, there isn't a control to edit the daily digests in that way. 

Each notifications section in your My Settings panel generates its own daily digest email. You can have as many daily digest emails every day as notification settings that are enabled for daily digest emails.

https://experienceleague.adobe.com/en/docs/workfront/using/basics/use-notifications/wf-notifications#daily-digest-notifications

@hannah_long  just checking in! Were you able to get this resolved? If one of the replies above helped—whether it completely solved the issue or simply pointed you in the right direction—marking it as accepted can make it much easier for others with the same question to find a solution. And if you found a different way to fix it, sharing your approach would be a great contribution to the community. Your follow-up not only helps close the loop but also ensures others benefit from your experience. Thanks so much for being part of the conversation!