I don't think the Crown being dark is down to the TV, but the programming. They master a lot of HDR content in darkness, so its not uncommon for it to seem dark. Some are better than others. The entire Disney collection also suffers from the same problem. If the TV has the backlight on full its working as intended.
With your in-laws, connect HDMI 2 from the TV to the sound base. After connecting up, and both devices are powered on, in the settings of the TV make sure the TV is set to output to the soundbar and not the TV speakers.
Last step is to make sure 'Auto Surround' is turned off on the TV because the soundbar will only receiver 2ch PCM and not 5.1 audio.
Useful parts of the manual:
The HDMI ARC (Audio Return Channel) function is called 'Bravia Sync' by Sony. So you want to read about that in the manual. The manual refers to an 'AV Receiver' but this functionality is the same as a sound base. Toggle on the options you want enabled like automatically controlling soundbar volume, powering on with the TV, power off with the TV etc.
If you have no luck setting up bravia sync, first be sure to you have the Auto Surround turned off, and if that is definitely off, try instead connecting the sound base via an optical connection instead.
The Sky Q box and Blu-Ray player should be connected also to the TV via HDMI, but not port 2. Port 2 must be the soundbase if you are using ARC to return audio back to the soundbase.