Coming from a similar car with the InControl (not Pro) with an Android so can offer my experience:
1) Things were much better when Spotify was an InControl App. Would allow you to search your lists, pick play lists etc. Unfortunately it went away.
2) I use Spotify exclusively. We have a Premium Account so can download music to the phone. Typically listen to a mix of what I have downloaded vs what is from the internet. Easiest thing to do is select Bluetooth as the source and control via your phone. Normally in that mode, next, back will work from the screen or steering wheel. I assume if you download music to your phone and use the native Android media player, should function in the same way. Have noticed that when streaming from Amazon Radio, typically starts from the beginning of the playlist everytime I start the car, but Spotify will resume.
3) System usually reconnects to BT when returning to the vehicle if listening to when I left. The situation you mentioned of stopping at a petrol station where the car may stay in Aux Mode maybe making things a little flaky, but generally not a problem. Thankfully the Android / BT issues got resolved. When Andriod 9 came out, the BT was broken and was enough to drive a sane person crazy as it wouldn't stay connected. I found a work around with a 3rd party address book connection that fixed it, but it seems to have been resolved with the latest updates on the phone (nothing has changed on the car but of course according to Android, the problem was the car).
4) The BT volume is a problem with BT and not the car. It's annoying. Make sure your volume is turned up on your phone as that can influence the BT volume depending on your settings on the phone. Can go into developer options and try to play with BT settings to up the volume but I haven't had much success.
5) I don't charge my phone off the USB port as that can cause confusion when trying to stream on BT. Stupid slow charging anyways.

Good luck