I ordered and customised this compression ring on Aliexpress. The headset is now 100% free of play (without any torque struggles, just as it's meant to be and ever was on any bike). It seems to be the Specialized design. I hope that it is also a bit gentler for the steerer tube.
referring to the video by henrychungfitness:

https://www.youtube.com/shorts/SIRHNtz9nvQ

https://www.aliexpress.us/item/3256805803796644.html

I had to widen the two openings a little so that the Shimano brake cables would fit through.

I cut off the outer plastic ring on the headset cap so that it doesn't rub on the frame.

4) Computer mount, which comes with handlebar is flexy, and I'm afraid it might break on bumps. Here some video only with garmin edge 520
[...]
And with 200g bike light
[...]
So I'm looking for more rigid computer mount.

You should be fine!
I've got the same feeling, but for now it is working fine with computer + light (~6 months of use).
Trace Velo has tested it and recommends the Hygge support unlike others on Aliexpress:
https://youtu.be/FERc_xzoESg?si=hbN-qjnUTMXCcVKx&t=102

I am confused by your statement about the cockpit being flexible.
People write a fair amount of positive feedback on the "the one pro", kecevlo, etc. handlebars which are all the same as the one you got.

Maybe the Lack of siffness correlates with your headset Player issue and your high spacer stack.

No, that are two different topics. There is play in the headset, I do not consider that as flex. The steerer is super stiff, the high spacer stack doesn't add a lot of flex (if any).

The handlebar is super flexy (I got the 380/120mm). You can already feel a slight flex on the brifters, but that's okay. In the drops I can bend it more than 1cm in any direction.
For sprinters this is not acceptable and I've never seen that on handlebars from reputable brands (neither alu nor carbon, they are all much stiffer).
A few years ago I had a Chinese carbon cockpit with a similar bend, it started creaking and I ended up breaking both handlebar sides right at the STI attachment point, during a load test.

The good news is that his cockpit isn't creaking yet, I'm not a sprinter, I'll keep using it (at least in the off-season).

Headset assembly:

I received the headset unit (pic headset_assembly_label.png) with cone A, but unfortunately, part B is missing. The problem is that cone A is not flexible. While the outer conical surface (blue) fits snugly into the inner bearing ring, the inner surface (green) lacks a positively interlocking connection with the steerer tube. An additional clamping ring is missing here, which could possibly press A and steerer tube together. Or cone A should be designed to be flexible to clamp itself onto the steerer tube due to its conical shape and the force from above.
I am not sure how cone B is designed, but if it is flexible, this might be the solution. A flexible cone is 'advertised' on hygge-bikes.com.
Another example of a flexible topcap:
https://ritcheylogic.com/bike/headsets/Switch-upper-headset
It's frustrating that such small details were messed up. Play in the headset (on a new bike) sucks! Luckily this should be easily fixable with another top-cap/cone. Top bearing and steerer are standard, but will the interfacing to the spacer dovetails (or if spacer are swapped too, to the stem) then fit?

Just received my frames this week and wanted to share my thoughts with you all. Got pretty much what I expected for the price, considering they came from China. No major design flaws that can't be addressed. Let's dive into the details!

Positive:
- The paintwork is really beautiful
- The frames are super light (1063g in size 55/L)

Negative:
- The seatpost clamp is a pain. The hole of the middle wedge is too small and the screw prevents the wedge from moving sufficiently towards the seatpost. No matter how much torque you tighten it with, the force only works against the edge of the hole and not against the seatpost. (I'll do a separate post about that)
- The handlebar is just one of the cheap generic ones from aliexpress. It feels extremely flexy, I'm not sure I trust this one.
You can often find it branded as Kocevlo, but there are many other brandings too:
https://fr.aliexpress.com/item/1005006028345937.html
Strangely, their website shows a completely different cockpit and headset assembly:
https://hygge-bikes.com/products/high-performance-integrated-road-handlebar
- The headset sucks and both of my frames were missing the top plastic cone. The headset has a slight amount of play. This is by design as there is no such thing as a compression ring/cone that could hold the upper bearing inner ring and steerer tube tightly together (different post).
- The frame and fork are very supple. I think they went to the limit, is this really still safe to ride? (I did a lateral stiffness test on the fork, it bends a lot -> different post).
- Seat tube angle: 73 degrees is too small (too much backset) in size L. Combined with the huge setback of the post and a big saddle pull (13cm mark, close to the maximum), it's really a pain. I am in the most forward position and still 5mm behind my optimal position (I really hope that one day a 0 setback post for the generic FM236 frame will be released).
- The front thru axle threads are only glued, other suppliers now show the fork with a replaceable insert
https://fr.aliexpress.com/item/1005005982170845.html
https://www.alibaba.com/product-detail/New-Product-FM236-Red-glossy-Hidden_1600862434667.html

Neutral:
- Headtube angle is quite slack and I was afraid of the bike being really sluggish, but in the end I found it sufficiently agile.
- The chainstays are not wider than on other (disc-)bikes I've ridden. So no problem with heel clearance for me, although my heels are close due to my slight V-shape foot position.

The build was quite easy, luckily on 12s Di2 I don't have any wires going to the STIs.
7.7kg (55/L) including pedals, bottle cages, Garmin mount, powermeter (heavy 105 11/34 cassette, not the lightest tires, ..) that's really good for an aero bike!!

Could anyone measure the chainstay height at the flat mount mounting point?
I would like to make sure that I get the right screw length for the rear caliper.
Thanks a lot!