Two things :
1. You can use the second HDMI output from your Pi to carry audio to your Soundbar, and the first HDMI output to carry the picture directly to your TV if you have a spare HDMI port on your TV (removing your sound bar from the video path)
2. If you are watching an HDR10 video without your TV in HDR10 (aka PQ/ST.2084) HDR mode, and thus watching it in SDR (but still in Rec 2020) - you will see very nasty pictures indeed - and instantly know something is wrong.
If you are watching an HLG HDR video in SDR mode but still Rec 2020 - you will notice no major differences until you get in to the highlights of the picture - which will just look less bright. However there is very little HLG out there (BBC iPlayer uses it, as does Sky Q)
1. That is what I have been doing but CEC is not working correctly when using both HDMI ports at once (10.0.2)
2. I've re-encoded on film and compared the output: they dont seem to be very different to my eyes but it could just be the opening scene. The HDR one triggers to the TV to Rec 2020 mode, the redone one does not. The HDR one stutters a lot (bandwidth or perhaps HDR10+?) where as the other plays smoothly.
Regardless, I've learnt a lot!