The package arrived a bit late but its all done now.
It works!
I got help on the technical side so don't fully understand HOW this works but got
4x LM-NP-1001-B1L Transformers
and soldered those between the RCA cables coming from soundbar to RCA mixer.
And everything works as it should. No audio lag and every format soundbar supports available but with upgraded stereo speakers.
Total cost for this project was 38€ plus RCA cables.
If somebody stumbles on this thread looking for this kind of solution for them, message me or reply here and I will go more in to details of this project.
Thanks everyone! I will be back once I start building my "real home theater" setup.