I agree with lots of bits of this, most of which are commonly aired points (like armour needing to be more effective), so I want to focus on a few where I disagree, partly. Threads can be more interesting and come to better/more nuanced suggestions if there's a bit of constructive disagreement
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B2 - player clan NPC parties are as functional as any other AI party. If you're at peace, they'll patrol, beat up bandits, train some troops and lose a bit of money. They're pretty useful for levelling up your younger siblings; at least if you're happy for them to learn certain skills party leaders learn. If you're at war, they can join an NPC army - or if you want to keep an eye on them (if they have valuable troops), bring them into your own - even if you have to dismiss them just as they get close, if you don't actually want to be in an army with slower party speed, and then invite them again as you move away. They're also useful for recruiting when you're busy and need to stock up on troops generally.
B3 - the two things aren't directly related of course. You're a blank slate at the point you're killing looters, so are learning quickly. By the time you've got 200+ troops and are at level 20+, progression is much slower. TW have said levelling and skill progression balancing and tweaks are in the pipeline; personally I think some of the issues with it are overstated - or are being compared to the speed at which
your clan levels up/becomes a superpower kingdom, which is ridiculously fast (and my biggest gripe with the game as a whole, as I think it has an insidious effect on many other elements). There's a change in
which skills will progress when you get into the mid-late game; leadership, in particularly and engineering to a lesser extent if you don't hire an engineer.
B5 - I really don't seem to experience the 'standing still' / unable to use towers AI as often as some do (or say they do); but if there is a pause at times, it can be overcome by giving an order. I do sometimes see a group stop at the top of a siege tower when they've dispatched the initial defenders. Order them to charge, and they charge. Obviously, can still be improved, but 'non-functional' is hyperbole.
B7 - True enough, but there is a 3rd way between idly being at the whim of an army leader or beating up soft targets: patrol alongside the army and try to force those interesting battles you want. Armies will often get fairly close, before one retreats or they pass by ignoring each other; a quicker party with the thought process of the player can usually manufacture a battle out of it if you want to.
B8 - Again I think, hyperbole. They do level; slowly. Initial companion creation (skills-attributes match up) had seemed to improve, but last couple of playthroughs I've noticed some badly imbalanced companions; maybe RNG. In my last game, my surgeon/healers (one died) made decent progress improving their medicine skill. In ~1200-1500 days I probably got starting level 14-16 characters to level up three or four times each. Not great, but reasonable if they're specialists not exposed to too much risk of death in battle. If they're front line infantry then yeah, until death rate is lowered, they'll die before they level up much. Caravaners I've got from 0 trade skill to 50+ (the useful perks for a caravaner are at 25, 50 then not again until 150 or 175, I forget which) relatively quickly, with minimal necessary attribute/focus points (2 and 1 respectively; some of those I've done it with I needed to level up once to reach even that minimum).
IIC - When you reach a certain point (certain quests, then battles vs lords), you should be capturing enough horses in loot to more than manage troop upgrades.
Completely agree that the trade skill needs linking better to running caravans, both in terms of caravans providing skill progression, and skill progression providing greater profit (even the trade skill of the companion running the caravan doesn't have much influence on profit). Leadership and steward need some rework; after a certain point, morale becomes almost impossible to drop below 100, while leadership takes too long to get started. Tactics could also be quickened up a little and some non-simulated battle perks given (but not all; I suspect quite a lot of players simulate encounters with bandit parties, etc. - particularly while death rates are so high).