Link for the stem? 

This one:

https://www.bike-components.de/de/FSA/Non-Series-SMR-31-8-Vorbau-p88038/?v=12039-black

Also you need the corresponding adapter for the acr headset cover (only if you go fully internal, you also have the option to go semi, with the hoses routed underneath the stem):

https://www.bike-components.de/de/FSA/SMR-auf-ACR-Adapter-p88118/?o=1000334301-black&delivery_country=DE&channeltype=pmax&upid=google&utm_source=google&utm_medium=cpc&utm_campaign=20543344239&utm_content=&utm_term=&gclid=CjwKCAjwwpDQBhAuEiwAa-4Wozp94RgC8hffViLy1SuXdOOGW9dH0PSEkVFaQJDilEaTW8VH0DWrFhoC8QYQAvD_BwE

And the Acr Headset (NO. 55):

https://www.bike-discount.de/de/fsa-no.55r-1.5/acr-steuersatz?delivery=DEU&currency=EUR&mtm_source=google_ads&mtm_medium=cpc&mtm_campaign=css_ph&gad_source=1&gad_campaignid=23176838830&gclid=CjwKCAjwwpDQBhAuEiwAa-4Wo3TaX9gL6Cex4M-1QewHIQuGZvJyuA7iDDs6kzFHOSTSzyra5tB31RoCunAQAvD_BwE

Hello from my side,

After reading and benefiting from all the insights here, I can now post a short middle-term review. Overall, I am super pleased with the bike and the build.

Here are the specs:

Size: 54 metallic frame (I believe it’s colour 3)
Seatpost: Zero offset
Headset/Stem: Fully integrated FSA Headset and SMR Stem
Handlebar: Balugoe, 36.5cm wide
Shifters: 105 Di2
Derailleurs: Ultegra Di2 rear and front
Cranks: Magene PES P505 Powermeter, 165mm
Chainrings: Spécialités TA
Cassette: ZTTO 11-34
Wheelset: Tagoola Aeroraptor (60mm deep, 23mm internal, UD-Satin finish, 5mm bladed carbon spokes, 1260g)
Tires: Continental GP5000 S TR, 30mm
Saddle: Fizik

The bike just fits like a glove and rides super well. I was skeptical if the wheels would be problematic in crosswinds, but so far they perform better than expected. The communication and build process went without any problems.
I love that so many details are now standardized: UDH, BSA BB, and the headset. I first installed a Kocevlo integrated handlebar but felt it was too soft; I believe most of the movement came from the top headset cover. After switching to the FSA headset/stem combination, it felt totally different. I now have much more confidence in the bike and the build.

Super happy with the bike, the only problem is that I genuinely don't know what to upgrade now!

So, my build is done 

Looks Awesome! which size did you get ? I am currently undecided between an S and an M and try to figure out which is visually more appealing to me..