Why have fewer spearmen than sheilds in the first three warbands?

That makes sense.

Here is what I see. You've got a lot of might but your heroes are not very strong duelists. At 800pts things like Boromir, Ambur, Gil Galad etc. start to turn up. The wraith and troll might your best bet for dealing with something like that but a stronger melee hero is worth considering. Even if it just using your troll to proxy as a chieftain to take the place of Gorbag. Or perhaps a black marshal on horse with lance to create multiple threats at different places on the field.

Keeping your banner with the man killing power of your army (the black guard warband) is good since they and the troll can make better use of it than anyone else but a second banner is worth it if you move the troll as described above.

Lastly, without any fast moving or shooting units you've only got on game plan and that it to rush the enemy. Assuming you've got heroes that can use heroic march that is good. You will be needing to do that in a few scenarios.