I mentioned I would come back if I learnt something.
So after updating my Nokia 7 plus with the Android Developer Preview 4 beta3 (the latest at the time of posting this). My phone connected to the LG soundbar SH5 perfectly with volume controls and everything. It even solved my bluetooth issues I was having with my cars media navigator.
So give it a try if you want, it seems to be nice and stable for me. I got the OTA after joining the developer program.