In the "history" of available modifications for non-Android Sony players, there are two types of modifications:
1. Tuning mods - these consist of modifications done to the SWUpdate.xml file (that is located in the files of the Sony firmware updater tool), and they started to appear sometimes around the end of 2019 on some Chinese forums, and then they were brought here at the end of February 2020. You can track down the discussions about this in the locked WM1Z/WM1A thread, as well as in the WM1Z/WM1A modifications thread.
2. Custom firmware - this consists of actual modified firmware, while using the stock SWUpdate.xml file content, and the first occurrence was in around June/July 2020
I created this thread for having a place where we can share feedback about custom firmware for the non-Android Sony players.
You can find all the available firmware modifications done by me on www.mrwalkman.com.
For any kind of questions/issues, feel free to use the contact information available on the website: www.mrwalkman.com/contact