@flyride
Thank you for your response. I have progressed now and found that the problem was multipronged. But you got me started on the right path, which is what I was asking for - so thank you!
Here's the breakdown of what I ended up doing:
1. First off, apologies for confusing the UEFI+CSM. I had not read correctly in the BIOS. It is either UEFI, Legacy or UEFI + Legacy. I have now set to Legacy.
2. This did not solve everything, alas. After some tinkering, I figured out that my BIOS does not take kindly to Legacy + "Boot USB devices first" as well as "Boot from removable media" - so I disabled these last two. Now the only items checked are "Legacy" and "Allow boot from USB"
3. With this, I was able to move on to detecting the DSM if I manually selected the USB stick from the boot menu (intercepting with F10) at POST. However, if I let it try to boot by itself, I got an error stating that there was no bootable device
4. After some rummaging around the forum, I figured out that it might be down to my (quite old) motherboard maybe not being able to boot automatically from a USB stick that used GPT partioning. And there was an .ing file of the 1.02b bootloader in the instructions for these cases. But I needed 1.03b
5. After some more digging, I stumbled across a post from @wimmetje (see inserted link below) where he took the 1.02b MBR image and replaced all of the files with those from the 1.03b loader, which worked for him.
6. So I replicated that, using the 1.02 MBR image, mounted the two FAT partitions and replaced the files with those from the 1.03b loader (modified with PID, VID etc)
7. I was now able to let the computer boot unattended, directly in to the GRUB menu - and after a little while, I could see it in the Synology Assistant.
I have since moved on to installing the 6.2.3-25426 PAT file and am exploring this brand new world The DSM even sees my add-on PCIe x1 4-port SATA card (Marvell 9215-based)
There is still much to learn, but I am on my way now!
Modifying the bootloader MBR image