@Wannes Fransen Thank you for reaching out to us, with respect this requirement - can we adjust the time that the backup takes place (not the frequency)? Or perhaps configure this that it's weekly on Sat/Sun?
Researched on this currently we don't have an interface within the portal to change the backup time of Entra Domain Services instance, however the timing of the backup can be changed by opening a case ticket further our support team will work with our engineering team to make this change.
If you have a valid support do open a support ticket, else share your Domain services details over the message so that I can assist you further on the same.
Will share my details over private message.