This is how I would prefer to do it, just for the main plots:
Epidosde 12:
All of the false parley between Sauron and Maedhros is concluded in this episode, if possible (if there's time). Maedhros is definitely captured in this episode. Morgoth interrogates him, which we don't show on screen. We do see Angband politicking between Gothmog and Sauron. There are more back-and-forths of messengers, and the Feanorians refuse the final terms Morgoth sends them (in Ep 12 if possible, very early in Ep 13 if necessary).
One of the last scenes opens with Fingolfin's host stepping onto real land, off of the Helkaraxe. They pause in relief and to get their bearings, and in that minute, the Moon rises in the West. We see some reaction shots of various Elves and some Orcs, but focus on and finish with Fingolfin's host. Their relief and joy, and blowing their trumpets.
The Moonlight may also be what reveals the Feanorian camp to Cirdan and Celeborn.
Episode 13:
Fingolfin's host are somewhere in northern Lammoth, resting.
The Feanorians fortify their camp at Mithrim, and declare Maglor King of the Noldor.
Fingolfin's host start moving southward and a bit eastward in search of food and somewhere warmer to rest. They get into northern Hithlum. They have a chance to rest and regain their strength. (We show that time is passing by putting other events [including events of the lesser plotlines] between this and the next scene with Fingolfin. We also have a Moon now, to help with time.) Feanorian scouts are out and about in Hithlum, and they might see Fingolfin's group at this point and report back to Maglor. Or, we could make it a surprise when the host gets closer to Lake Mithrim (which would be my preference).
The Sun rises in Act 3, allowing some time between Moon and Sun.
When the Sun rises, we see reactions from a lot of people: Fingolfin's host, Feanorians, Falathrim, Sindar and Melian in Doriath, Nandor, Orcs, leaders in Angband, Maedhros on Thangorodrim. We see flowers blooming and other plants sprouting, and animals waking up.
Fingolfin's host see Orcs fleeing northward because of the Sunlight. (There could be a skirmish so Fingolfin's host knows these are definitely hostile. I can imagine they're wary of accidentally starting another Kinslaying against the first strangers they meet, just because they're ugly.) Fingolfin's host follow the hostile creatures north to Thangorodrim (which is visible as soon as they get past the nearest mountain range). They go all the way to Angband and smite upon the gates and blow their trumpets. Maedhros hears them, but nobody can hear him. The bad guys in Angband are rather in disarray at the double arrival of the Sun and a new Noldorin army, so every evil creature hides in Angband if they can get there.
Fingolfin realizes they can't just defeat Morgoth like that, however, and his host are not at all ready for war. So he turns his host south, towards the warmer land with the mountains surrounding it.
As Fingolfin's host moves through Hithlum, his scouts report that they've found a camp with Feanorian banners waving over it. Fingolfin marches them right towards the Feanorians. Maglor moves them to the other side of the Lake, trying to avoid conflict. We show the Elves of each group discussing the other group. We emphasize the tension and anger between them, so that the audience doesn't know whether war will break out. Part of that rising tension is Fingolfin forging Ringil. Is that sword for war with Morgoth... or for conflict against the Feanorians?
We end with the Men waking up in Hildorien looking up at the Sun and around them at the world in wonder and delight. Grunting and pointing and laughing, rather than singing or inventing words right away.
There are the Dark-Elf side-plot(s) to fit into Episode 13 as well, concerning Doriath, Thingol, Cirdan, Eol, and the Guest-Elves. I think their last scene has to be Cirdan himself reporting the arrival of the Noldor to Thingol, which means we can show his journey from Mithrim to Doriath as a signal of how much time is passing during this episode, after the Moonrise.
The Hiding of Valinor would be near the end of the episode, but not the final scene.
If we end with Fingolfin at Angband, then the difficulty I see is the timing that leaves us with for Season 4 Episode 1.
If we need to make this series finale 2 hours long, we might be able to convince the Hosts to let us. However, Episode 1 of Season 4 will be 1 hour and needs to include Fingon rescuing Maedhros, so I don't think there'll be time in S 01 Ep 04 to get all the way from Fingolfin's trumpets at Thangorodrim to that. Putting that all into one episode risks giving the impression that Maedhros is only there for about a week or two at most, and it was no big deal at all, which I don't want to do. Most of all, I think Fingon couldn't to come to his decision so quickly. I want to give him time to think about it, to find out what happened to Maedhros, and to rest and recover after the ordeal of crossing the Grinding Ice. Fingon is supposed to rescue Maedhros in Year 5, not immediately after arriving in Mithrim. If we finish Season 3 with Fingolfin in Mithrim, then those 5 years can pass between Seasons, off screen. That seems far easier to me, and I want to avoid making more changes to the timeline that we don't need to make.
Timing in the next season is already going to be awkward because of the large time period, and possibly extremely cramped if Nick can't persuade Corey to give us 2 seasons before Beren and Luthien. We can't assume Corey will change his mind about this. It has me very worried. It's quite possible we'll have to somehow squeeze 465 years of mementous events and character development, including Dagor Bragollach, into only 13 hours of television. That would be extremely difficult, and probably require skipping numerous important historical events. (If Dagor Bragollach takes up an hour, that leaves 1 hour per 39 years.) I strongly believe we need to plan ahead in case that happens.
So, I think the best option is to avoid adding additional events to Season 4, and not to end Season 13 before Fingolfin gets to Lake Mithrim. Tension between the Noldor is a suitably dramatic cliffhanger ending, and finishes some scenes so that we won't then have to give short thrift by force-fitting them into Season 4 Episode 1. It also trims Season 4 a bit, by letting 5 years pass during between-season downtime.