If your TV doesnt support HDR10 or Dolby Atmos, HDR metadata shouldn't be transfered to your TV. Instead using your ARC directly, use your TV directly instead and see if it made any difference
Connect Fire TV output HDMI to your surround reciever (Ex: HDMI 1 Input) then connect the reciever output to your TV. Then go to your Amazon Fire TV settings, under Display and Sound settings, check Dolby Digital Plus Automatic instead Default.
You can use your phone as a remote but youll need an 3.5mm ir blaster or built-in ir blaster to control your TV. Some devices doesnt support 3.5mm ir blaster so check your device if it supports 3.5mm ir blaster before buying one.