Debian/Ubuntu on Synology DS214play

Hi all! I'm new here, what an amazing forum!

I have an old Synology DS214play ('Evansport'/Intel Atom CE5335 CPU) which I am trying to get a 'normal' Linux install onto. A hw teardown can be found here FYI:
https://www.techpowerup.com/review/synology-ds214play/5.html

I have so far managed to connect to the serial port, navigate my way around the boot loader (Intel CEFDK) and burn a bootable Linux kernel/initramfs to Flash. I am using a small kernel as a secondary loader for a 'full' boot as there is only 8MB of Flash - nowhere near enough for modern kernels.

So far I have managed to get a small OpenWRT x86_64 generic kernel to load with a minimal initramfs containing just busybox, insmod, tftp client and kexec. This just about fits in Flash :) [NOTE: I can't use the stock 32-bit kernel as it does not support kexec and I've no idea how else to chain load another kernel.]

Once this OpenWRT kernel is loaded I can insmod the e1000 ethernet kernel module, TFTP a 'full' kernel/initramfs to ramdisk and kexec into it. For now my chosen 'full' version is Ubuntu 24.04 LTS which I run on other machines in my home :) This I have booting OK, and running with an NFS rootFS i.e. as a diskless workstation.

I have no 'special' Synology kernel modules loaded i.e. no 'synobios.ko', so i cannot presently access internal SATA drives or the SDCard or USB2.0 front panel slots, nor control the LEDs. The rear panel USB3.0 slots work via an external Etron chip which works fine. I have not tried the eSATA port but I expect this also to work OK as it is also via an external Silicon Labs chip which I see load into the kernel correctly.

What do I need to do to get the internal SATA drives going? I understand from my research that there are some GPIO settings required, but no idea how to do this.

Let me know what info you need ;)

cheers
Stu

PS: Apparently this Intel Atom CE5335 CPU will happily run 64bit binaries, even though the stock kernel is only32-bit. I have no idea why Synology did not support 64-bit on this CPU? This is one of my main motivations to upgrade!

PPS: there is some shenanigans going on with the hardware serial port - I had to set my kernel cmdline baud rate to 14400 and it actually comes out as 115200. Also I get NO output from processes on the serial console, only kmsg output appears. Infact if the runtime processes do try to write to the serial console, they hang for 30sec. Odd...

Quick followup - the LED/FAN commands given here:
https://forum.doozan.com/read.php?2,139072,139197#msg-139197
seem to work, but via /dev/ttyS5, not S1. So that's promising!

Hoping with your help I can get the SATA drives going...