Thanks again for all the ideas and recommendations above guys!
Update: after a lot of looking I found ES-3 & ES-6 available (from 2 different vendors). I think that will be a strong solution for me because:
-- lightpipe (ADAT) I/O to my interface will spare me the "2 interfaces" issue noted above, and I have spare ADAT lanes on my interface to use for that. It should be a tight integration
-- I don't need a ton of I/O between the modular and DAW, I'm hoping ES-3&6 will be enough, but I could add some more "to modular" I/O with ES-5 and expanders if needed. The "to Daw" signal maxes out at 8 channels with ES-6 and its expander. I imagine I could get a 2nd pair of ES-3&6 if I really really needed more I/O, assuming I had the ADAT lanes to spare.
-- I like that ES is a premier brand in this space. I'm hoping by going with a trusted provider that I will have no stability / synch problems.
-- Motu828 (mentioned above) seemed like another plausible solution. But ES-3&6 costs less and has fewer hard to access (rear panel) connections for my particular setup. So I would consider that going forward if really needing more I/O, but for now ES-3&6 seems like a better pick for me.
I may be back with more questions / comments. One thing I've not figured out yet (and haven't tried because my modules haven't all arrived yet) is how to synchronize DAW & modular transport / clocking? Let's say I'm sending transport "play/stop" signals from my DAW, and clock from my DAW. That goes to modular, gets my clock/sequencer chain going. Then modular is sending audio, CV and gate back into DAW for recording. I'm hoping to synch/slave that all so that it's not a total chaos and mess; and so I don't have a ton of cleanup to do on the recordings in the DAW (e.g. if a lot of recorded tracks ended up off the metrical grid due to lack of clock&transport synch). Of course, suggestions on these points would be welcomed.