To be honest, I would not look to be spending £500 if you are only going to be playing mp3 files in the car. You simply won't hear the benefits of an expensive player capable of playing HD music. Also, I guess portability is not a crucial requirement either. At the budget end you might want to consider an android tablet. These typically have good sized batteries and if the wifi is switched off, can run several days on a single charge. I have an Amazon Fire HD8 tablet and the battery on this lasts for ages. If you want to use HD files over wifi it can deal with those too. Same with Tidal. You can actually download Tidal HD files to the device at home and play them back in the car over BT. Another option is something like the Moto G7 Power phone. I recently acquired one of these because I was fed up with my phone "dying" at the end of each day. The Moto G7 Power will literally run for days providing you are not playing videos over wifi all the time.
Coming to specialist mp3 players, I can recommend the Sony range. I have an earlier model but this model due to be launched soon looks as if it will meet your requirements (long battery life and good quality bluetooth). It will also perform well with good headphones on HD music files. https://www.amazon.co.uk/Sony-NW-A55...qid=1572440056