In fact I don't really have any problems with systemd support for xorg, but I am a bit worried for the direction things are going.
I perfectly understand that this is not any problem *now*, but based on what has happened with other projects (like udev and GNOME) I am worried about the direction things are going.
There's ways to make PA play nice with Jack, sure. The way it works is either that Jack runs all the time and PA uses Jack as a backend (instead of using ALSA directly), and that lets normal applications use PA, while actual audio applications use Jack directly. Or, you configure PA to suspend when you start up Jack, and restart afterwards, so that they're never running at the same time.
PA 4.0 had a bug fix for "graceful" handoff between pa and jack. Also, since you're using a daw to produce the music I wouldn't call that playback but music production.
Beginning somewhere around version 2.05 or so, I ran into sessions refusing to load. I rolled back to version 2.0 for weeks until I noticed that after an update that failed to load at runtime, .xsession-errors showed the session crashing over the absence of cinnamon-screensaver. The session should not have been written to fail on that, the fix would be a custom session if for some reason you could not install the screensaver-of if the session in the future refuses to load in the absence of Pulseaudio. When Cinnamon sessions won't load, Cinnamon will still run by sibstitution for another window manager or starting X directly into it.
It would suck if I have to fork Cinnamon-session for my own use, but it should not be too difficult if it comes to that. This is what I like about open-source software: if you don't like something or know a better way, you can change it to suit your needs. I still remember an issue back in 2009 with Audacity where sliders in the compressor didn't work, so I downloaded the source code, changed the numbers for the compressor's defaults in the source to the ones I always use, and recompiled. This worked until the sliders were fixed and my hand edit was no longer required,