As I want to start building one good army next to my evil armies, I was looking for some fluffy options, ending up with the garrison of Osgiliath:

Warband:
Boromir, Captain of the White Tower with Banner of Gondor and shield
7 Warriors of Minas Tirith with shields
7 Osgiliath Veterans with spears and shields

Warband:
Faramir, Captain of Gondor with heavy armour and shield
7 Warriors of Minas Tirith with shields
7 Osgiliath Veterans with spears and shields

Warband:
Madril, Captain of Ithilien
5 Rangers of Gondor
6 Rangers of Gondor with spears

How workable would this be as a semi-competitive list? I could swap the last warband with Beregond and some mounted citadel guards with longbows to get in some fast moving units.

Idk to be honest. The warriors of Minis Tirith and veterans have some perks that will play well as a holding force for your heroes to do the killing and your archers are numerous enough to be a threat and defend objectives well. If you played this with a solid infantry block with Boromir in the center, Faramir on a flank, and used the rangers for board control it might work pretty well. That is assuming you were fighting someone that would be willing to give you a straight fight in the center rather than trying to sweep the rangers and surround you by playing the whole board.

I might suggest mounting Faramir and giving him a warband with knights and rangers while resigning the second warband, after loosing two guys, to a captain or Cirion to help Boromir. Given how crummy the knights are though that might not be a tangibility improvement to the board control aspect. You could also try replacing Mardil with Damrod and using some of the rangers points, along with the difference between the heroes, to get an Avenger to force the enemy to come to fight you in the center but the rangers might be enough for that already.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

Thanks, that's some sound advice.

Just when I was about to tweak the list, I saw the new Wardens of Gondor profiles. I think adding in Ingold would greatly benefit this list since it makes sure my warriors will still be able to benefit from the shieldwall rule in combat by not having to fall back and thus disconnecting with the rest of the frontline. Therefore, I came up with the following list:

Warband:
Boromir, Captain of the White Tower with Banner of Gondor and shield
6 Warriors of Minas Tirith with shields
7 Osgiliath Veterans with spears and shields

Warband:
Ingold, Warden of the Rammas Echor
5 Warriors of Minas Tirith with shields
6 Osgiliath Veterans with spears and shields

Warband:
Faramir, Captain of Gondor with heavy armour, lance and shield on armoured horse
6 Rangers of Gondor
6 Rangers of Gondor with spears

This way I maintain my bow count, while mounting Faramir will somewhat improve my board control. I could consider some knights, but as you said yourself they're quite squishy.

EDIT: I could also swap the rangers for 6 mounted Citadel Guards with longbows so as to improve my board control.

I could, however, also go with the Avenger so as to force my opponent to fight me where I want him to. That would look as follows:

Warband:
Boromir, Captain of the White Tower with Banner of Gondor and shield
6 Warriors of Minas Tirith with shields
7 Osgiliath Veterans with spears and shields
Avenger Bolt Thrower

Warband:
Faramir, Captain of Gondor with heavy armour, lance and shield on armoured horse
7 Warriors of Minas Tirith with shields
7 Osgiliath Veterans with spears and shields

Warband:
Damrod, Ranger of Ithilien
3 Rangers of Gondor
3 Rangers of Gondor with spears

I like the final list best but I've got a soft spot for the Avenger that may not be entirely justified. One the bright side though, the lists are so similar that I do not see a reason why your collection could not include all the variation for you to alternate between. Two boxes of warriors, a box of rangers, a box of knights, an Avenger, and then the heroes will give you a good number of models and quite a few options when building for smaller point games too.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"