Paul,
Update on the 24bit/192khz FLAC issue. I completely cleaned and reformatted the card as you suggested, put the music back on it, checked it for playback off that card on a computer (no problems) made sure all the tags were clean and there was no album art whatsoever, and reloaded the album on the iBasso DX80 - no change in behavior, it hung immediately rather than playing the track. This is on the latest firmware FYI, 1.5.8.
Just to see if it was something that could be corrected by switching firmware, I also tried Lurker's latest firmware based on 1.5.8 and had the same problem, so as that was no the issue I went back to stock firmware.
After playing around with other albums I noticed the issues are very album specific. Take three albums, all FLAC 24bit 192khz legally purchased from HD Tracks, and all of which play fine on VLC player from my laptop even if the data is being transferred from the iBasso itself.
Crosby Stills Nash And Young - Deja Vu - will not play at all and crashes as previously noted
Phil Collins - Face Value - will play but skips and stutters periodically (and extremely frustratingly)
Louis Armstrong - What a Wonderful World - seems to play fine with no real stuttering or other issues
Again, all three are the same format purchased from the same well-known website, yet they all play differently on my iBasso DX80.
It would be awesome if someone else could try these albums on their DX80 and see if they can replicate the issues, but asking someone to pay ~$75 for music so they can help test a bug is a bit unreasonable.

Still, nothing I do seems to have any effect whatsoever on playback, and it's not a consistent problem with all 24bit 192khz music, it's very much track by track. It's worth noting too that I've tried playing these albums at this point on three different MicroSD cards with exactly the same issues, so it does not seem to be card specific. Any thoughts would be greatly appreciated!