I took a quick look at the settings feature on the Volumio App. Can't see the equivalent to the features you mention below in Moode.
Before splashing out on DAC/Digi add-ons it would be worth checking whether you have maximised the USB option from within Volumio.
I use Moode and have bumped up the USB output on a standalone RPi quite noticeably by simply tweaking some of the settings.
Would have to plug it in again to check exactly which but from memory I definitely increased the buffer size and this made the biggest difference in terms of minimising stuttering and raggedness. I think I also set SOX resampling to 24/96 (but this is what I would need to check)
In terms of DAC HATS I find IQaudio very good - I tried Allo Boss and found no improvement although costing nearly twice as much.
I have also used Justboom and this was also adequate if edged by the IQaudio DAC.
I have yet to try and alternative PSU and use one of the Raspberry own brand - noting that these supply 5.1v which is apparently critical ( YMMV )