I'll keep this post updated with my findings and reviews.
Unfortunately, I've noticed that powerful USB DACs (so not the cheap dongles) have regular popping issues on the Pixel 6. This has not been fixed even with the June updates, but reports suggest using the DAC as a dedicated device through a supported app (Tidal or UAPP) doesn't cause such issues. This is still something I'm investigating.
Devices tested: Pixel 6, 3a
DAC model: Apple USB-C to Jack adapter, EU version
The Apple dongle has very good measurements, and for only 9$ it can outperform DACs with a much higher cost. Needless to say, people don't need more unless they need more amplification power for their low-sensitivity headphones.
While it works with Pixel devices, the internal volume of the DAC is very low on Android and cannot be changed, therefore maximum volume will be quite disappointing on all Android devices. I wouldn't recommend it unless you have very high sensitivity headphones, in which case it could be enough.
Also, note that the EU version is underpowered compared to the US version.
Devices tested: Pixel 6, 3a
DAC model: Samsung USB-C to Jack adapter, WW version
The Samsung dongle doesn't have the excellent measurements of the Apple dongle but it still provides very good performance for its low price. Also, it doesn't suffer from the same volume issue as the Apple dongle, so it might be a better all-round choice.
Similarly to the Apple dongle, the WW version is underpowered compared to the US version.
Devices tested: Pixel 6
DAC model: UGREEN Hi-Fi Audio Pro
Usually sold for around $50, this DAC dongle has the very good CS43131 chip. It might actually be a rip-off of the Meizu dongle, but I'm not sure of that. I expected it to perform well, and while it sonically did, it was prone to terrible EMI noises when using LTE on my Pixel 6. Since my other dongles don't suffer from that, I returned it. Could have been a defective unit or the design of the dongle itself (shielding, etc.) was not up to the standards.
Devices tested: Pixel 6
DAC model: iBasso DC05
A bit higher-end than all the options above, it's usually a solid option under 100$ and packs way more power than the cheaper dongles aforementioned. It has the ESS Sabre ES9219C chip, which performs well. But beware that since it's more powerful, more battery drain should be expected. Also at that price, I would have preferred a detachable cable.
Devices tested: Pixel 6, 3a
DAC model: Chord Mojo (v1)
The Chord Mojo is a very high-end portable DAC that used to be sold new for around 500$. A bit overpriced if you ask me. It packs a lot of power though and has physical buttons to control volume. It suffers from poor design choices, such as the microUSB port, and the fact that it always runs on battery.
No issues with the 3a, but as mentioned above, there is a lot of crackling/popping sounds with the Pixel 6.
Devices tested: Pixel 6
DAC model: Qudelix 5k
This is probably the most polyvalent DAC I've ever tried. It serves mainly as a Bluetooth DAC/amp (there's support for LDAC and aptX HD codecs), but also as a USB DAC. It has powerful features such as integrated PEQ filters, all of which can configured through a neat Android/iOS/Chrome app. It uses the same ES919C chip, sounds great, has a balanced output for those who need it to drive their low sensitivity headphones. It also regularly has firmware updates and their support team is very responsive.