First.
Day 700 is to early for an end game event. This could probably be a difficulty option where 700 is on the hard game while day 1000 is normal and later is easy.

Second
The other issue is that there are 2 ways i've seen or heard that works. Autoresolve or soloplay with a mount. Trying to play and win with an army is pointless.

My solution.
Make it that wight hordes without a walker don't grow in size. This make the stacks beatable by grinding them down. This makes sense as it is the walkers that keeps the wights "alive", once they are gone the horde crumbles. By assasinating all the walkers you can stop the growth and win the endgame. Its still tough but doable.

Third
Diplomacy when WW invade. All factions attacked by WW should sue for peace and ally against the new threat. As it goes the WW is a myth and fighting them is probably not a priority if the Iron throne is up for grabs. But once the walker/wights attack your realm that should be your priority hence declare war on WW and sue for peace with all other factions.

Fourth
WW should probably attack castles and raise them to the ground (not realy what you want as a player but it´s probably more thematic). Not wight hordes tho.

I Like this mod and I will play it again