But there are a range of mixers on the market, not as beautiful, from Zoom (F4 and F8) and SoundDevices (MixPre 6, 8, 10) that have integrated A to B format converters. With the SoundDevices also having a binaural renderer integrated into the headphone out.
The larger issue is the one you have already identified, the lack of pre-amps. IIRC the field mixer they sell does contain preamps and DSP, but unsure if it can provide 48v that most Ambisonics mics from any manufacturer will require.
EDIT: In looking at the site for the field mixer they make [0] it appears it doesn’t have pre-amp except for a headset input. I was confused because their large diaphragm condenser that they have coming out will absolutely require a pre and power to operate, but it looks like it has an 3mm out with integrated battery operated pre-amp in addition to XLR and 48v phantom. [1]