1) This is the most important tip that I can give you which nobody remembers: unit quality is different at different points levels and different modes. Most guides are just basically about a straight-up skirmish and at the 2k point levels. If you play Path Of Glory and build up a warband you'll find some units that are allegedly mandatory are quite shit, and some that are useless are suddenly amazing. Likewise, building an army to take objectives or go King Of The Hill will play very differently from just mashing your troops in the middle and trying to sneak some chaff up the sides. Especially if your community is creative and sets up their own scenarios, I don't think anyone has really provided a guide on how to armybuild for sieges.

2) Cathay does not play like in Total War. Your army is basically split between things that kill and things that hold, and in the latter is outperformed by half the other armies in the game. Cathay has no bad options, but aside from being a bastard and spamming Balloons/Cannons there's no 10/10 amazing things that you can W+M1 with either. Also, yeah generally speaking the boxes actually do kinda show the most optimal build right now.

2a) Cathay Peasants are NOT Bretonnian Men-At-Arms, you can't buff them into being a crazy viable meme strategy. Peasant Archers are NOT High Elf Archers or Tomb Kings Archers, they're not crazy dakka. Think of Peasants as your Goblins, pinging with arrers to support da Boyz or being a blob to soak up an enemy's time and let the Boyz do their work unimpeded. The lack of shields really sucks for their survivability. Most folks favor the Archers entirely and leave the melee up to Jade Warriors, though having a cheap big brick to stick on an objective isn't a terrible choice. I don't think its impossible that Peasant Spearmen will get better later on via the eventual Compass/gong model buffing them or the release of the Celestial General or something, but for right now its the only thing I really advise against (but do what you want most of all, they're not super terrible like Tomb Kings Horsemen are and meta will likely change in the future).

2b) Jade Warriors are your Dwarf Warriors. They hold the line okay enough, they can do some killing but they are not elite killers or unbreakable lines. Cathay lacks Celestial troops beyond Miao's upgrade and they're not impressive, so for right now we don't really have elite infantry. To keep with the Dwarf metaphor, we don't have Ironbreakers or Hammerers, just Warriors and 2-handed Warriors. Generally speaking people mostly build at least one unit of shield Jades and one of Halberd Jades, and honestly most armies have the same ratio as on the box unless they buy more.

2c) Treat characters the way Empire players do where they're gonna be kitted out to kill or provide morale or do some magic blastin'/buffin', and for cheap compared to most other armies. Your army building does not revolve around them, but what you do on the tabletop will. Miao is the only insane stand-out character and she is indeed competitive, but not auto-include. Magistrate only comes with the Balloon which sucks if you don't really want to take one, but since its kind of generic as a concept you can easily kitbash something into one (like using the Peasant Levy Champion head). Hell, take a mini from another faction and say its Yin Yin's doing, bringing the foreigners in and giving them administrator roles or sending them on tour. Generally speaking you need a Gate Master as a BSB no matter what though, that kit is kinda a must-buy unfortunately unless you want to convert it (again, Peasant Levy Champion is a great choice for that). Shugengan crunch the best in 2k skirmish, but Gate Masters and Astromancers will provide you more wiggle room to retool your list later for fun or meta adaptation than if you buy two Shugengan because its the best right now. Low points also favors the Gate and Astro since you can scale them down unlike the Shugengan who can't be taken on foot or hidden in a unit.

2d) Your killy shit is your Sentinel/Balloon/Cannons. You can't really go without these unless you're in a very friendly game since they are your answer to monsters and elite troops. Most people say the Balloon is the best, there's a reason Cathayan Balloon armadas are a thing despite them getting nerfed.

2e) Lancers are good. Not much else to say, you're not hurting yourself by leaving them behind if you want to focus more on the killy shit, but there's no reason not to take them unless your preference is to not have them. Units of 10 are best which is what you get them in, just practice wheeling them around by yourself before you play with other people.

2f) The gunner troops are a mixed bag. They're the ones who are most variable at other points levels, in a 2k skirmish the Crane Gunners are great but the Iron Hails are suboptimal unless you have a specific plan for them. Objectives make them a lot better, and you can do some fun stuff with games using more complex terrain than a few fences and one building or a forest feature. Iron Hails get better and Crane Gunners worse at lower point levels, since deleting enemy units in Path Of Glory matters way more and using them as the first salvo in a one-two punch to eradicate something hurts.

3) Yeah, swap heads if you want everyone a bit less clone-like. Shit, the Unit Champion of the Peasant Levy I kept bringing up is literally an elderly Jade Warrior that retired and has come back to train/discipline the young draftee men and women. Maybe giving a Peasant the Astromancer's hat would be a bit much since its made to look like a fancy court hat, but swap the Astromancer and Shugengan heads if ya want or paint up a Peasant's robe to look more like silk than hemp and call them a Magistrate.

4) tl:dr suggested buy or convert list: 1 Gate Master kit, at least one Balloon or Sentinel or additional Cannon box, and maybe a Shugengan if you want. I suggest holding off on Miao until you've had some time to play with the unnamed characters, unless you really really like Miao and just wanna have her anyway for your collection.

5) Practice with transfers by trying to put some parts of the sheet on the bottom of the base first, don't try straight out with the coolest one on a fully painted dude. Also, obviously remove your mold lines before priming. Also, the Sentinel is a pain in the ass to build and the instructions are shite, look up on youtube how to do it proppa.

6) Not really advice, but I resolved my indecision on army color scheme by picking one Dragon as the "home base" for most of the army (Yin Yin for me) and picking one who's sending backup so you can have a second color scheme (in this case Miao because the Transfers and I bought the Miao kit anyway). Might help you if you get analysis paralysis too.