Wow these are some really good recommendations actually! I looked into the Dengfu R12 and that looks like a crazy beast. The only unfortunate thing is that it's slightly pricy.

For anyone wondering what parts I am getting:

Frame/Fork/Handlebar/Seat post: Tan Tan TT-X34
Groupset: Shimano Ultegra Disc R8020
Bottom bracket: Shimano Ultegra SM-BBR60
Power Meter: PES P505 Base Power Meter
Pedals: Shimano Ultegra R8000 SPD-SL Carbon Pedals
Wheels: Elitewheels SLR-DISC 2.0
Tires: Vittoria Corsa Pro G2 TLR
Saddle: Pro Stealth Sport Saddle

If anyone is curious about how the build turned out let me know 

It's been a while since I received the frame but was only able to build the bike 3 weeks ago since I was waiting for the Ultegra DI2 upgrade kit to become available as I did not want to buy a complete groupset and end up with a BB and crankset that I will not use (crank length 172.5- I use 170 and PF BB). Wheelset is Elitewheels 65mm.

The build was super easy as am experienced building and servicing bikes and every part fits easily and how it's supposed to be.

To address the main issues discussed in this forum, seat post slipping and headset play, I have to say that I did not experience any seatpost slipping, which is mainly due to upgraded seatpost clamp with horizontal striations that lock the grippy part of the seatpost very well, and the mandatory use of carbon paste. As for the headset play, I did some mods to the C-ring. These consist of applying aluminium tape on top of the C-ring to increase the thickness by 0.3 mm (the same can be done with microspacer but I did not have any, so tape it is) and also 3 layers on the inside where the C-ring interfaces the steerer tube. I think that the C-ring needs to be 8,2mm total thickness (stock is 7,9) to avoid headset cover rubbing on the frame. If I did not add the tape, it would have rubbed after torquing down the top cap to 2.5-3 Nm. Another thig is to use carbon assembly paste on stem/steerer interface. After the initial ride the play returned but I did not torque enough initially (only finger tight to take up the slack, which is not enough obviously). Another thing that I done is also applied thread locker to the nut part of the stem bolts since there was none there and the bolts were rattling themselves loose (torqued to 6Nm only to become finger tight) after only one ride. After the mods of the C ring and top cap tightening and thread locker to the stem nuts there is no more play after 500 km with deliberate riding over cracked roads.

The supplied garmin mount acts as a leaf spring vibrating the hell out of everything attached to it. So far it has costed me a GPS unit (IGP sport, perfectly fine) which vibrated to disintegration, and yesterday my light had shorted out due to the vibrations. Use aluminium mount which may be sturdier than this thing.

The ride feel, as mentioned by other riders is subjective, but compared to my Cube Agree GTC Pro is waaaayyyy better and the bike is stiffer, a lot more responsive, rails corners, fits me a lot better and added 2-3 kmh to my average speeds instantly. Cruising at 45 km/h on the flats seems effortless or at least a lot easier to maintain. Thanks to @TidyDinosaur for advising me on sizing - this thing fits me like a glove (175.5cm 82 cm inseam and went with size S) with full stack of spacers to begin with but as I become more familiar with the bike I will remove 1cm of stack.

Total cost of this project 2500 eur. Can't beat that with what you get for this price compared to western brands.

P.S. for some reason I am unable to post with attached picture. I was only able to add this post as a quick reply. Any ideas of what might be the issue?

You can't know much about aerodynamic efficiency by just looking at something. You need to test the stuff. Look at the nose of an F1 car, it's a very boxy shape.