This is already possible. Set Mobile Maxiumum birate to what ever you desire, just not original or no transcoding will take place.
If you connect via VPN to home server use
*VPN connection as mobile
if you also want to treat public wifi as a mobile connection you need to set the wifi as a metered connection on your clients wifi settings and set “Metered WI-FI connection as mobile” in symfonium.
This way you achieve
Outside lan = transcode.
Inside lan = no transcode.
Ive set my server to transcode to opus by default, I dont understand why you would want MP3 over opus either. Opus is superior even on low bitrate.
You can also use Symfoniums built in transcoding engine.