I've built the O2 as well, and loved it, since for the first time I could drive the HD650; it even has enough power to properly use them on a flight (don't do it, you'll go deaf). But the form factor just isn't that great for on-the-go. Plus with the batteries installed there is no space for the ODAC, requiring a DAC solution (or a phone with a good DAC).

Give the FiiO btr5 a try instead of spending too much on a schiit, or look for similar devices from other manufacturers. A lot of them can do both USB (96kHz/24bit) and decent (LDAC) bluetooth. I could not tell a difference between a friend's BTR5 (dual ES9219C directly driving the headphones) and my Q5 (dual mono akm4490, ad8620 gain stage with dual op926 buffers when used balanced). Of course my hearing could be too damaged^^'