I didn't have much fun with jack either so ended up using alsa-loop instead. So in my set up the HifiBerry DAC ADC Pro receives an analogue line in feed, this is picked up by ST, ST then outputs into Alsa Loop. Liquidsoap receives the audio and sends it to Icecast.
Yes the GUI definitely introduces an overhead, but as things stand with the GUI, ST, Liquidsoap, Icecast all on the same box, there's no buffering issues or any kind of CPU throttling so far, but I'd like to eventually move Liquidsoap and Icecast on to a different box to relieve the CPU of load. It is working rather hard and the CPU temperature varies between 67-70c at the moment. Still within spec and I believe throttling tends to occur at 80c and above. Running 24/7 so far without any hiccups nor crashes.
I'd give it a go with GUI first with the 64-bit OS and see how it goes. Just make sure pulseaudio is configured to match the sampling rate you set on ST and on the encoder (you should be fine if you're using 44.1khz, otherwise you need to check it's at 48Khz or whatever you chose)
I'm definitely not going back to 32-bit. I've managed to reinstall everything without issue. You could create an upgraded build on a spare Raspberry Pi if you have one, and maybe swap over the SD card during the graveyard shift or perhaps when it's the holidays.
Have a look at my long thread when I started getting into all this, Minetti was super helpful with his advice and tips and did mention a trick with Jack you could use to send audio point to point between Raspberry Pis.
viewtopic.php?f=2&t=32372
You'll find it towards the end somewhere. I actually got ST to recognise Jack and got the link to work. But I had trouble selecting the jack inputs correctly on ST so had to pause it for now. You might have better luck.