To become Archmagister, Aryon has given me the brown book and I should secure the support of all councillors. But they have no topic about that even with persuasion at 100.
Is it because my stronghold still is at stage one?

EDIT: It seems, Aryon has skipped the "recruit a mouth" quest as well... after Shishi he promoted me to magister.

This is a little odd but if you're really magister you should be able to kill Gothren and become archmagister I think. Make a forked save and try it.