Iphone watch sync with text messages

I don't know where you get that messages delete automatically as new messages are added, that has yet to happen for me.

I delete as I go on my Watch.

Deleting from the Watch does not delete from the phone nor does deleting from the phone delete from the watch.

Only sending and receiving are syncd

Unfortunately, not yet. The Messages on your Watch delete automatically as new Messages are added. This function is independent of any action you take on your iPhone or iCloud.