Hey Phil !
2.1Amp is enough to boot safely. Most powerbanks do that and more now, to support fast charging, for devices that use ‘fast charge’. I wouldn’t wanna go lower.
Just make sure to use a high current 22 AWG USB-to-2.1mm DC cable (with 2.1mm to 2.5mm DC plug adapter to work with the USBridge).
I think Allo said continuous draw is 1.4A which seems conservative but if we assume it’s about right (especially powering the USB WiFi adapter also) then you should get over 10 hours continuous playback easily with your 20,000 mAh powebank - assuming your USB DAC is not bus powered.
Give it a try! There’s something different to the sound when you completely eliminate all potential for ground loops and leakage current loops through the USB input. And this is a low powered USB endpoint close to the DAC which is always nice.
I’m also getting the metal case and 1m USB3.0 extension cable just to further separate the wireless adapter from the gear.
Cheers! Sean