After Varnhold Vanishing I went straight back to my Barony and did some Kingdom Management to wait for The letter from Brevoy to start hour of Rage but it never turned up, is this a common bug?

You have mostly forgot to talk to Meagar Varn a second before leaving Vordekai tomb. Since there is no ' You should not pass', he can be miss and the main quest is then halt until you go talk to him.

He is at the secret exit, the one you use to quickly leave the tomb at the end.