Technically there is a mark as read option. You have to go to the server you want to mark all as read, press and hold on it, and then click the mark as read option. For DM's however, you can only mark as read when you get a notification of a DM. Unfortunately, I don't believe there is any other way to mark DM's a viewed other than actually viewing it.Hope that helped!
This doesn't work for me on my Google Pixel 3a