I have finally soldered it and it works pretty well so far:

Once connected via USB to a PC it announces four UART ports:
- Port 1 is for JTAG (I don't know if and where JTAG pins on EOS RP are and if they work on 3.3V level. But if they do, I can use this with tools like OpenOCD for example.)
- Port 2 is reserved for FPU and is not connected in this first revision
- Port 3 is connected to cameras ICU RX/TX pins via 1.8V voltage level shifter ICs
- Port 4 is directly connected to cameras 3.3V MPU RX/TX pins
dmesg output:

In practice connected to an EOS RP it looks like this...:

... and this (ICU on the top, MPU on the bottom):

However, it requires some advanced SMD soldering skills to get this assembled. Some PCB manufacturers like JLCPCB do offer a SMT assembly service, where all SMD parts can be ordered presoldered together with the board. That way one can just order the board plus the cable and can start right ahead without soldering at all. Therefore I am working on an upgraded version where this is possible.
I have also made a few mistakes in my DigiKey order so I have to fix the BOM in my github repo. Once this is done I will publish all schematics and gerber files of this.
Signal quality is good. I cannot observe any noticable noise, ringing or glitches:

Voltage for 1.8V ICU line is perfectly on 1.8V:

All in all I am very happy with the result.
May the UART adventures begin.
