Top right.

Mark all as read

Yeah, the relays don't keep track of read messages. It is a client level handling. Just got to the 3 dots and Mark read.

Don't worry. I gotchu. Two years ago after a large spam attack, my DMs were wrecked and unusable on all clients. Howeverz our amazing devs heard my cries for help. Most, if not all clients now have a mark all as read feature. Thanks spammers! In Amethyst is under the 3 dot action overflow menu.

Instructions unclear. Sending you a bunch of DMs