Well I believe everybody will give you a different opinion. I believe first of all, you should consider what and how you will be playing using RPi and volumio. If you are looking to play MQA, then obviously you will hardly find an option for an internal DAC supporting it. Bear in mind Volumio is NOT a MQA decoder as opposite to Roon or Audirvana. Then, you should consider whether RPi will be the only one source you would have. You might have other players you would like to us the DAC with then, again very difficult to find an internal DAC supporting that setup.
Addressing the SQ, internal DAC or Digi HAT w/o own clock and separate power supply will not provide SQ any better than the USB port on RPi 4. In contrast, it will introduce limits in bitrate, so if looking to a HAT or internal DAC you should bear this in mind so you do not get “the worst of two worlds” (no better signal but other limitations).
I believe it is overseen by most of the people that it is possible to “clean” the USB signal with simple (passive filter e.g. iFi iSilencer) or more complex solutions (e.g. iFi iPurifier, or Matrix Audio X-SPDIF 2, both reclockers, rebalances and active noise filters) so you technically have low jitter, cloned signal similar to the Allo Signature Player but still the benefit of using USB w/o bitrates limits or even I2S for external DACs supporting it.
I have tested many combinations (excl.Allo Signature, unfortunately) and ended up in my simple setup with a RPi board running Volumio (or Roon sometimes, I admit) USB connected to the Topping e30 via the iFi iPurifier, both independently powered by Allo Nirvana PSU. This setup is quite affordable and has produced excellent results with respect to jitter and SQ, very comparable with my other setup with AURALiC Aries. As a matter of fact it sounds by far better than any other HAT setup I have tested (excl. Allo SIganture or Boss and Kali, which I have not had the opportunity to test, but I recon will be very comparable in terms of SQ, but with bitrate limitations). I also admit, I do prefer an external DAC for that setup anyway since I am using it also to connect a CD player…