[NathanX]

HDR to SDR is set to automatic, have also set it to Enabled (or on), rebooted, but still the same dark result with washed out colours.    

And other options "Limit display color depth to 8-bit" and "Use 4:2:2 color subsampling" also give washed out colors ?    

Switching between 4:2:2, 4:4:4, 4:2:0 YCbCr shouldn't change anything unless there is a bug in CoreElec/AMLogic's kernel - they are just different ways of carrying the same content (4:4:4 has no colour subsampling, 4:2:2 has half horizontal chroma res, but full vertical chroma res, 4:2:0 has half horizontal and half vertical chroma res)  RGB vs YCbCr may expose some oddities - but nothing earth shattering - again unless there is a bug.

8-bit limitation may allow additional HDMI output modes to be used I guess - but again - I'd be surprised if that changed the EOTF/Gamut tone mapping approach used - again unless there is a bug.

(For info 2160p50 and above RGB and YCbCr 4:4:4 output is only supported at 8-bit.  For >8 bit you need either 4:2:0 YCbCr at 10-bit or 12-bit, or 4:2:2 at 12-bit - as there is no 4:2:2 10-bit option in the HDMI spec) 

Thanks a lot for the technical explanation - i did as Sholander suggested and yes you are right - nothing changed. A pity.
So it seems that I will have to wait until hdr to sdr tonemapping will improve... maybe Kodi 19 will help there.