Wow. First post I can actually contribute to!
Yes, I have ordered a keyboard from keebart. I ordered the 3w6 and asked on his discord for some custom changes. He helped me with the customization and gave me a few options to choose from.
I wasn't really too precise in what I wanted but keebart was really good in giving me exactly what I wanted.
Really happy customer. I picked it up in person and the response times on his discord are incredibly quick.
As far as I can tell at least assembled in Germany. Cases are 3D printed by the seller.
The seller is a German local and approachable. PCB's are ordered from somewhere but I can not tell you from where. Might be European might be Chinese.
Seems like a one person company and the German market for custom ergonomic low profile split keyboards isn't too large.
So if you are interested just sent a message and ask yourself! His discord is active but as far as I can tell E-Mail works as well.