Peter -- Thank you for the reply. I will try Redpill. I need to study the install procedure. Please do add RS3413xs+ to the supported configs.
Right now I have experimented with various older bootloaders on a thumb drive and I can get the Synology restore assistant web page to load and with the older 5.x versions, I can even get it to say DSM is installed but on reboot, it never boots into DSM, only into the restore assistant.
One basic conceptual question that I am unclear on -- when I install DSM via the restore assistant, I must have a separate sata drive installed. However, I never actually see any files on the sata drive. It appears console log a bunch of updates on that first reboot, so perhaps the install files are being written to the thumb drive? I have set the VID/PID properly.
Since there is an official Synology linux kernel, should I be able to swap the zImage file in, after I get Redpill and DSM working?