Relic will not develop the future titles. Ad you said, World’s Edge hired new graphic director with Unreal Engine experience.
Relic owns its Engine": Essence. It’s a good Engine but old at the same time because It not supports many modern features like DLSS.
About next developers my suggestions could be: Forgotten Empires (they have more than 200 developers) and Tantalus.
There are also other RTS developers. Personally i never thought to Relic for AOE. I mean the announcement was a surprise.
So, we Simply don’t know.
Hovewer i tried to explain also my toughts about a future of the saga here (It’s a very long post)