Surprised you dont have Aux, in my 08 XS its under the armrest in the storage box. theres a 12v socket next to it which is handy, I have a bluetooth adapter plugged into Aux powered by a usb adapter plugged into the 12v socket.

An FM Transmitter might be an option for you (Ive never had or tried one) - they pick up bluetooth from your bluetooth device and convert the signal to FM radio signal that can be played through your car radio. As I say ive never had one so I dont know how good they are - maybe have a look at some YT videos to get an idea.

Have a Subaru forester 2018 xt, owned from two years old mint condition full Subaru service history with 40k miles. Factory fitted removable tow bar used twice for small trailer - time for a change looking at £19000

how much fluid does it actually take - do we know?

See above for possible solutions. I've had mine for over 4 years on a 2019 (70 plate) OB, no problems (but now I've just tempted fate!). Hope you find a solution.