Maybe Morgoth breathing fire into Glaurung could be shown at the beginning of Episode 13 without it being clear what is going on until Glaurung himself starts breathing fire later in the episode.Right - the goal would be to tease some stuff in the background a few times.
If someone from Valinor is knocking on the doors of Angband (so to speak), the inhabitants would be very suspicious and distrustful. So, I think Maia!Glaurung's arrival has to either happen while Morgoth is in residence (only he could vet the guy satisfactorily), or he has to know someone from way-back-when and be let in for that reason. Showing Maia!Glaurung persuasively talk himself inside Angband sounds promising. So, perhaps this could be a scene some time in Episodes 1-6?
Maia!Glaurung would be a background character in the Episode 8 'Meanwhile in Angband' stuff.
The conversation between Morgoth and Maia!Glaurung over the silmarils? and Morgoth's desire to make use of him in the war can be an Episode 10 tag.
We should probably save the 'transformation of Maia!Glaurung into a dragon' scene for Season 5 in flashbacks.