I recently got a 2018 Lexus and was annoyed by this as well. There is no Bluetooth setting on the vehicle to turn off auto-play on connect. I did however find a solution for me which involves my iPhone. I know nothing about an Android phone solution.
On the iPhone use the ‘Shortcuts’ app to create a ‘Bluetooth Automation’ shortcut.
Go to Shortcuts app → Automations and create a new Automation.
Select Bluetooth, choose your vehicles bluetooth ‘Device’, then the ‘Is Connected’ option and set ‘Run Immediately’ and hit ‘Next’
Select “New Blank Automation”, then scroll right to select the show ‘Media’ actions.
Scroll down to ‘Playback’ actions and select ‘Play/Pause’, then select the ‘Play/Pause’ action itself and select ‘Pause’ from the dropdown.
Select ‘Done’ to create the Automation.
You should now have an Automation that will ‘run immediately’ when connecting to your vehicle Bluetooth and perform the ‘Pause’ function. Sometimes there is a blip of audio before the Pause is performed.
This works for any active audio I have on my iPhone at the time (e.g. Plexamp, podcasts, any other audio app on the phone). Hopefully this helps iPhone users who are also annoyed by auto-play.