PCs these days often still have an optical toslink, that can be used to avoid the issue.
I'll just deal with it by keeping the O2 volume low and cranking the volume on a second amp. Long term I'll just buy an element or a schiit.
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^^'
The Fiio options do look good. My current desktop application has me switching headphone and mic between a work laptop and personal desktop and headphone and speakers often. I use analog switches, but I'm leaning towards a one box solution in the future rather than adding more hardware. So something like a mayflower ARC mk2 perfectly covers all of my demands (including a bass boost option for my open-back headphones). I already use a USB switch with a powered USB hub, so really it would be swapping out 3 boxes and many cables for one box. The schiit hel 2 looks really great but I've read reports of 200 ms latency.
