In explaining the Emperor’s fleet, I think the best place to put it would be when it is established.

“What could you…offer me?”

“Everything. My Empire was not defeated…it was preserved…frozen at my command. And now…awakened. The might of the Final Order will soon be ready. It will be yours…if you do what I ask.”

Then Poe could reiterate:

“He’s been planning his revenge. His followers have been building something for years. Upgrading a fleet of Star Destroyers that vanished after his death. He calls it the Final Order. Each ship has been modified with dreadnought cannons. The Emperor and his fleet have been hiding in the Unknown Regions on a world called Exegol.”