Another solution could be to make war more profitable. All alliances seem to want to sign as much treaties as possible with as large alliances as possible just to be safe.
There is no benefit in taking risks or reason to oppose other(larger) alliances other as'making the game 'more interesting, there are only benefits(protection) in trying to ally with them.
I dont now how this could be corrected. An possible option would be in-game peace terms. For instance surrender in a war by giving away infra, tech, land or 5 even happines. It would give alliances an (economic) incentive to try to win wars and would give nations/alliances a reason to try to overtrow dominant alliances.
(but I realise, if done wrong it would just have the opposite consequence by giving dominant alliances more instruments to dominate other alliances )