my er9 included all imaginable bolt lengths and adaptors for 140 to 160mm discs, something that costs extra and requires significant brain damage on Shimano. I think a very small fraction of the market cares about post mount (at least on the road setups), i wouldn't even want to pay for the shipping cost of bolts of another thread size. Shipping from China is a large portion of the cost of everything we ride in the West.

Yes they do include a generous collection of hardware. Unfortunately a) I asked and was told they were post mount and b) the calipers are using an 70mm spacing that looks like post mount but uses M5 bolts like flat mount. Something I read tonight suggested that this is one flat mount standard (the ones on my road bike are much narrower at 30something mm) The included adapters are also 70mm rather than the post mount standard 74mm (seriously, who in the industry decided to invent a new flat standard 4mm different and using one size different bolts? ) I thought about drilling it out but there isn’t enough material in the right places. I also do not find anyone doing flat to post adapters for the 70mm flat variant.

And while you’re right about drop bar bikes being more likely to use flat mount, this is a gravel group set where post mount is not rare.

So, now I’m wondering what calipers might be compatible with the LTWOO levers. I know that early on they were bundling with other makers calipers, so I’m guessing the piston ratios are probably common. Maybe Shimano compatible since they went with mineral oil. If so, that would be great since apparently pretty much all Shimano calipers are compatible including across road and mountain groups.

Not familiar with the name Delihea.

I note the shop listed on AliX on Oct23 only. Their business licence says ... ELECTRONIC commerce, established Mar21. So...
There are a few YT videos of people reviewing Delihea bikes, from much before 2023. From people buying on AliX.
Can't find a website for Delihea.

the ck-two that takes 32C looks VERY interesting. But that frame ticks all the boxes for 2024, i'd say.

I dont think i've seen this frame before. The other one with 28C looks much less future proof and relevant in 2024.

Can you explain how does adding an extra spacer will resolve the headset play? Also where did you purchase the spacer from?

I've been seen to 3d print moon shaped spacers so that i could compress the steerer column, avoiding the plastic thing that sits just on top of the frame, from rubbing on it. If it rubs, some of that compression torque is going into the frame, which will probably result in headset play. Generally, nothing on a bike should rub.
It probably sounds theoretical, but once you've built a few bikes and faced these problems, it makes sense.
As w most things, you learn by doing.

Make sure you torque enough & use carbon paste.
If it's a tolerance thing, then consider using a soda can shim, or some fiberglass cloth.
Also consider greasing the thread of the plug, especially if you've sweated on it. The torque shouldn't go into turning the screw, the torque should go into pushing the expander into the seat post.
Afaik, save for crazy exceptions, "all" seatpost slipping is fixable.