I am thinking of doing the following:
I already have a RME Digiface USB with 3x ADAT expanders, with a total of 24 analog inputs and 16 analog outputs.
If I’m in the studio, I can connect the Push 3 Standalone to the remaining ADAT input and output and use TotalMix to route any of the analog inputs/outputs to/from the Push 3. When I leave the studio, I will connect a compact interface like the Motu Ultralite MK5 to the Push 3 so I can connect more instruments.
I would be mostly in the studio, trying out different combinations of different external instruments combined with the Push. When I finally decided on a good setup to use live, I can just disconnect those instruments and leave all cables in the studio, bring them to another location, and reconnect them but with the Motu interface instead. If I keep the same instruments on the same input numbers, I think I don’t even have to change anything in the routing settings of the Ableton project that is inside the Push 3 standalone? The only thing that may be different is the converter/interface latency, but this is probably within 1 ms, so not really a big deal.
I think this must be perfectly possible, but I may have overlooked something?
Also, when switching from a standalone project to Ableton on my desktop in my studio (or vice versa), I would have to change the audio outputs of the External Instrument devices (since I now use the Digiface as audio interface instead of Push 3), but I can easily do that by creating presets for each instrument for both Standalone and Studio modes.