many thanks for your input, that went by a bit ungratefully neglected by me... I apologize!
For the sake of anyone still considering using the same setup, I want to share my experiences here :
I have been successfully using my Atomos Ninja Star with an Atomos S2H (converting SDI to HDMI) converter for the last year now.
It is admittedly awkward having to convert SDI to HDMI for the Ninja Star.
Mainly because HDMI connectors are just not made for professional on-set use.
But I found the overall form factor of the recorder and converter combination very attractive and have a good spot on my Betz-Rig sled, underneath my monitor for it.
The automatic recording function is working very nicely with ALEXA, Alexa Mini and Amira cameras.
To enable the SDI trigger on these cameras you have to turn ON internal "Audio recording" in these cameras! There will be no SDI trigger on ARRI cameras otherwise.
But please inform your DITs beforehand, for them to expect some empty audio-tracks within the recorded files of the steadicam.
I did occasionally use RED Dragon and Helium, Sony F55 and Panasonic Varicam cameras with SDI triggering, too.
We always found a way to make it work, but I do not remember every individual menu setting unfortunately.
And we always shoot solid 24fps or 25fps here in Europe. So I did not try 23,98!
The build quality of the Ninja Star is good.
Atomos H2S converters though are prone to several problems! I had one issue with the tiny switches not working anymore and then a soldering issue with its battery contacts!
The 1920 x 1080 ProRes 422 recording of the Ninja Star has good image quality for demo reel purposes and it easily survives some "grading". (I hope my DP´s are not reading this.. ;-)
All of this wisdom might be obsolete by now anyway, as Atomos discontinued production of their S2H and H2S converters !