Hi Geoff,
Nice that you already have the ADAT parts. We use it in the office for parts of our audio (broadcast company) but for music I'm so humble to stay with USB and jacks :-) Now you mention the shortage of ES-9 it made me double-check because I want to order one for myself. Fortunately, it's in stock in one of my favorite shops.
I'm also looking at integrating external equipment with modular (who isn't?). The core will be my iPad Pro. With AUM and the multitrack recorder, it's quite powerful. I just bought a Teensy to set up a headless Dirtywave M8. Teensy is via USB, but the M8 itself is MIDI and audio jacks. The ES-9 would then be key to get the audio in, but I'd still need to find a way to sync clocks and start/stop. No PC, because that turns everything into a complicated mess. I've got lots to explore, but I'm doing it step by step. I also just started recording video for my YouTube channel, so I'm already doing everything at once.
As for Disting and similar modules (looking at you O&C), I stay away from them. It's true - you can turn them into anything. But I find that confusing. A VCO is a VCO and not a clock, in my setup. But they exist for good reason, so spend your time to get to know it well because there's plenty in it!