[Jackaudio] JACK/ALC1220 issues

Hey, new subscriber here. I've been using JACK for several years but know just enough about it to get it to work (usually). I've hit a couple of issues with a new system build that have me stumped. Specifics of the system: Ryzen 3700X; Asus Prime X470 Pro motherboard; using (or at least attempting to use) the on-board audio, which is an ALC1220. OS is Ubuntu 20.04 LTS; JACK is configured to run with realtime priority. Issue #1: Can't get any output from the digital (S/PDIF) port. I've made sure S/PDIF is unmuted in alsamixer, and the S/PDIF output works fine with PulseAudio. I've tried outputting to all of the system ports that show in the JACK connection manager, and none of them produce any output on the S/PDIF port. Issue #2: If I run with the default number of output channels (6), JACK stutters horribly with constant XRUNs. Already tried changing the --period option; if I increase it JACK fails to start, and if I decrease it the stuttering gets worse. Also tried changing --nperiods; increasing it prevents JACK from starting. If I disable the last 2 channels (i.e. run in 4 channel mode) the stuttering goes away. I can live with #2 (don't really need the last 2 channels), but if I can't get #1 resolved I'll be looking at a 3rd party soundcard (internal or external) for this system. Any thoughts/insights would be welcome. Thanks!