Personally "It left the phone" state is very important to me, esp., when I am travelling and network connectivity is intermittent.

Also "Message Read" is a very ambiguous state according to me - If the content of a message is read from the notification drop down, but user never opened whatsapp to read it - is it read, or not ?

Definitely. You want confirmation it got to someplace same until delivered to the recipient. 1 check for got to server, 1 check for delivered to recipient device, and then "Read" underneath the furthest part of the conversation that has been read. Or I'm just crazy and niche.

I'd say: clock symbol till it left the phone, one checkmark for delivered, another one for read.

Perfect!