Thanks for the reports on the R4R and the other suggestions.
Some follow-up thoughts...
The R4R is a bit smaller than the Tascam Dr-100 mkIII, but the latter has the standard handheld features (screen/file management, plug-in power on 1/8th in, built in mics). The dimensions are probably similar if you get the optional attachable mics on the R4R.
R4R - 4.76" 2.76" 1.42" 9.7oz
DR-100 mkIII - 6.02" 3.15" 1.37" 10.24 oz w/o AA batteries
MixPre 3 - 6.53" 4.65" 1.40" 19.84 oz
R4R internal Li battery - 8hrs no phantom, 7 with Phantom power on mics drawing 3 mA according to the CEO (yes - great corresponder!)
DR-100mkIII internal Li- 12 hrs no phantom power, 6 hrs phantom power, according to the manual.
Regarding the MixPre 3 USB C connections: thank you all for the suggestions. Apparently my problem WAS the included Y cable, which just wouldn't stay attached to the MixPre 3. I was finally able to test it out with a USB A to C direct from my laptop.
I'm still mulling things over. All good options. I'm tempted to return the MixPre 3 and go for the R4R or a handheld like the Tascam. After playing more with the MixPre 3 I think I'd like to go as small as possible (and, in particular, avoid external USB power) on the second XLR rig. I still have my Zoom F8.