3.5mm, I now know why conventional consumer audio devices don't use mixers, they're a ground loop party

It's also that I don't know how to build a differential input not limited to power supply range, I do think they are sold and I have a mixer at home but it's somewhat bulky for my computer desks.

The CS5213 appears pinout-compatible with the MX929x, interesting. The AG620x is a chip I've had the misfortune of encountering; it was common in Amazon DACs, dropped signal output every 15 minutes or so unless you disconnected the EDID pins from the (monitor?), and interprets input signal in a cursed fashion where 16 or below produces black but only 255 produces full white. If your computer outputs limited range HDMI the analog signal can't reach white, but if your computer outputs full range HDMI the shadows are clipped. This is one of the chips that gave HDMI DACs a reputation for black crush and caused CRT communities to recommend DP.

Interestingly the MiSTer game system community actually patched their FPGAs to output digital signals from 16-255, and have some other way of avoiding the signal dropouts (never talk to EDID?), so this chipset is almost seeked out over incompatible models.