So this is the part 3 of my blog post to see part 2 visit:
https://lovelybabylon.noblogs.org/comprehensive-guide-how-to-use-mackie-onyx-1640i-with-debian-linux-on-lenovo-thinkpad-t420-with-near-0-xruns-at-48khz-with-around-20-322ms-total-roundtrip-latency-part-2

Another ALSA developer Takashi Sakamoto contacted me via email because he saw my messages on ALSA mailinglist. So I was asked to test new driver (kernel module) for devices based on OXFW970/OXFW971 chip. So it turned out my Mackie Onyx 1640i is using this OXFORD 971 chip (while some models are based on TC Applied Technologies DICE chipset family chip using snd-dice kernel driver instead). So I compiled and tested his code but it didn’t work at first few tries. The audio was playing in slow motion, distorted or you could hear only clicks and pops. You can read more about the issue [here]. Apparently there was a regression in kernels v5.13+. Also with the versions v5.13+ the “Ricoh FireWire patch” no longer applies smoothly. So please make sure to use kernel older than v5.13. And my suggestion is to you use Takashi’s improved driver on the top of kernel v5.12.0-rc3 instead. So you take advantage of both worlds. His work is most likely going to be a part of kernel v5.16.

At the end we managed to fix the issues with audio/video. I need to say the driver is way more stable then stock kernel driver. It runs almost XRUNs free all the time. Even with 64 samples 3 periods, that is 1.3ms block latency! Thank you Takashi! Enjoy!

Thank you for your time and I hope it was helpful.