Apart from the more regular changes I applied, what really made it click for me was letting go of the Expert colour temperature I had selected initially, switching to Warm, and turning the Color to 44.
I'd like to share a little update on this.
When one researches X90L recommended settings, there is really not much disagreement when it comes to color settings for this TV. General consensus seems to be that the X90L is remarkably accurate out of the box, after setting it to Custom + Expert 1. All other settings, like brightness, contrast or motion are either personal preference or also largely shared across the on line experts' recommendations.
So far I have only tried to optimize the DVB-C cable TV input, and never quite found myself agreeing with this. Also, as mentioned previously, the HDR channels didn't appeal very much either.
Until I switched to the Apps (Video) input, followed this setup advice again and put on some 4K SDR and HDR demo material in the YouTube app.
Man o man. I am completely blown away. Both SDR and HDR clips look absolutely stunning, and definitely more than accurate enough for me. Beautiful colors, with spot on saturation and excellent skin tones. Combined with its great black levels, my new TV gets me basically all I could ever wish for.
So please ignore the settings quoted above. I am floored by this TV's OOB performance in Custom + Expert 1 mode, and have to conclude that it has never been the TV but rather the DVB-C input's signal that needs correction.
Hope this is of interest to some.