Black Marshall, horse
12 Orc Trackers

Gorbag with shield
6 Black Numenorians
6 Orc Warriors with spear

Mordor Orc Shaman
6 Morannon Orcs with shield
6 Morannon Orcs with spear

That takes me to 476. I have no idea what to spend the last 24 points on.

My plan is to form a block with the Morannons, Shaman, Gorbac and Nazgul with the Trackers nearby. The Numenorians with their higher F and C can form a flanking block on their own, supported by spears.

My first thought for the 24 was to replace the 12 trackers with 7 warg riders.

I want to avoid just all Morannons, which is probably the "best" army since I want some model variety.

Any suggestions on tweaks, how to spend the last 24 or adjustments to get more points to add a 4th warband?

Well the easiest thing to do with the extra points is give the spearman shields, or if you don't find that favorable with the regular orcs give them bows so they have either dual purpose or higher defense. You could also put a couple of trackers on wargs.

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Who ever said swimming was easier then running never swam a 400 I.M.

First thing to do: Mount the Dark Marshal!
Second is propably mount your Shaman if you have the model.

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MEMM I: 4th (Mordor+Harad) MEMM II: 4th (Galadhrim) MEMM III: 2nd (Thorin's Company)

Forgot to list the horse, Marshall is mounted.

Why mount the Shaman? Won't he channel fury and hide for the whole battle?

Morannons are obviously very nice, but variety is worth something too - those Fight 4 models with Terror can definitely come in handy against some enemies, and at times, those Morannons may be the worse choice (much less effective against F4 Defence 7 Dwarves).

For the remaining points, I would put the Dark Marshal on an armoured horse (extra survivability, especially against S2 archery). Then either give some or all spearmen shields (starting with the Morannons, they're worth it more to be protected..) and spend the few remaining points on upgrading a bit here an there - or spend all the points that way, depending on what you want/have and how much of it.
Black Guard could be rather useful to have a couple of, while Spectres for example provide some fun and tactical options. If you're going down that way, I'd start with replacing Black Númenóreans, as neither they nor whatever they are promoted to can benefit from the Shaman's Fury. Or: simply upgrade the Mordor spear support to Morannon spear support. Defence is useful when shot at or the frontrank has gone, and the S4 can still be used.
Not sure of the warband organisation by the way, I'd rather have the Marshal near some combat troops to inspire, although I suppose he could move there soon enough anyway (as the other heroes should be in or near the frontline as well).

If you could, I'd suggest trying to find an extra point somewhere to get a banner, as at this points level you can start to include them without really affecting your numbers, plus you already have three full warbands.
I'd say keep the Black Numenorians, I've found the terror and high Defence and F4 means they can hold their own against a fair amount of stuff.

The Dark Marshall counts as a 6" banner.