Hi IG-88! It's an honor to have you addressing my question, as I have learned so much from your posts/replies in the last months. 🤩 Thank you for your support! 👍
After few days battling this issue, I gave up troubleshooting it and installed DS3615xs with Jun's 1.03b loader and the latest DSM (6.2.3-25426), because I had a good experience using the 6.2.2 version before (and I didn't need transcoding in this NAS box). I was trying to use 918+ simply because it is running in another NAS at home. Now, with 3615xs, everything is working great after few edits to adjust the number of drives and activate SHR in the synoinfo.
To answer your questions, so maybe it will help others:
I may have messed up the number of zeros on the HEX code, but I tried so many times, using different numbers from 12 to 24 drives (and different HEX codes for USB and internal ports every time) that I don't believe this may be the issue.
To reach 20 drives, I used 2 PCie Sata controllers: one for 6 drives and one for 2 drives (plus the 4 SATA onboard).
Sorry, I cannot provide the dmesg file simply because I installed DS3615xs and did not backup the DS918+ system files (as they were giving me trouble).
Now, I am trying to work with those ata ports that are always listed as DUMMY in the dmesg file (#5, #6, #9, #10). They remain empty. I also don't understand why disks 13 -> 16 are not automatically populated, as they are not listed as dummy in the dmesg file and I did change the grub.cfg file with SataPortMap=462 (as per my setup). I understand that I can use sata_remap (works great!), and I am also trying to learn more about SasIdxMap. Let me know if you have any idea in how to deal with these drive numbers that don't populate automatically.
Thank you again for helping this amazing community! 😃