It still amazes me that it's almost the end of 2018 and still no one seems to have been able to, to my knowledge, find or create such a device. I did however gather some more information, so here I go.
To clarify what the adapter should be able to do:
The USB-C male port is for the host device, aka a phone.
The USB-C-PD female port is for power. So it could be connected to a wall adapter with a C-C cable and provide power to the host device (and maybe also for the second device).
The USB-C data female port is for USB devices that can be connected to the host device, aka a flashdrive, mouse or 3.5mm dongle (with DAC).
For reference, I own a USB-C hub that does all these things, however it's very large (especially for a phone) since besides these ports it also packs in an extra USB-C port and two USB-A 3.1 ports. I'm talking about the Aukey CB-C48 https://www.aukey.life/products/cbc48-usb-c-hub-2-port-usb-3-0-2-port-usb-c-1-port-usb-c-with-pd. This device existing, shows that the idea of a "splitter" is actually possible, just never done before (to my knowledge).
The size I'm looking for is around this size: (Don't be fooled by its appearance, the adapter in the picture only wires the lines regarding power one way, while it wires the lines for Analog-Audio-over-USB-C the other way, which is great, if you have a phone with a DAC and only want to use it for headphones but not for anything else.)
Another option would be this form factor: https://hackaday.io/project/26263-pd-buddy-wye. (This one works the other way around, allowing your phone to fast-charge over PD from a charger and connect to a PC via USB. It doesn't allow the phone to be the host.)
Finally I found another adapter of which I do not know whether it works: https://www.amazon.com/gp/product/B07KSZMG5J?pf_rd_p=1581d9f4-062f-453c-b69e-0f3e00ba2652&pf_rd_r=SMDQYBNA8X75H9EGVK16. These types of adapters have a DAC inside them,thus enabling phones without a DAC to connect to USB-C headphones (or dongles) without DAC. At least that's what I've gathered from the descriptions. However, I don't know if the port can also be used for other USB peripherals. The seller did tell me it didn't work with mice/flashdrives, but it did work with the 3.5mm dongle from Google, which is odd, because it does have a DAC inside and thus is a completely normal USB peripheral, just like a mouse for instance. I did order one to test what's true and what isn't about these. When mine arrives, I'll update the post.
Edit 1: Another item I found was the Cusby, a failed indiegogo campain: https://www.indiegogo.com/projects/cusby-first-usb-c-modular-expandable-adapter and http://www.cusby.com/. In particular their "power port" module does exactly what I'm looking for. Unfortunately they've been silent for 3 years now, so yeah...
So if anyone finds anything, please let me know and if anyone has the capacity to design and build such an adapter also let me know. I really hope it can be realized!