I wasn't really planning on this upgrade but stumbled on a new 'Synology' branded ECC SODIMM for regular DDR4 money. I need more RAM than this but for now it does, at least, double my RAM and brings me into dual-channel mode. My search for a bigger and better memory configuration that escapes the new vendor waring in DSM will continue.
Usual dull and unremarkable Synology box:
Identical to the SODIMM that came with the RS1221+ but with an extra 'Warranty' sticker... serious stuff:
Not much to say about fitting a stick of RAM in acres of space, apart from Synology leaving microns between the RAM and the heatsink (why do I think @Rusty is laughing at the size of the CPU cooling):
"Call that a heatsink?"
Anyway, system booted-up and RAM checked:
Interesting that the max RAM is stated as 64GB.
Back to the GUI and everything looking ok and now rocking 8GB of RAM like it is 1999, again.

Usual dull and unremarkable Synology box:
Identical to the SODIMM that came with the RS1221+ but with an extra 'Warranty' sticker... serious stuff:
Not much to say about fitting a stick of RAM in acres of space, apart from Synology leaving microns between the RAM and the heatsink (why do I think @Rusty is laughing at the size of the CPU cooling):
"Call that a heatsink?"
Anyway, system booted-up and RAM checked:
Rob@Rivendell:/$ sudo dmidecode -t memory
Password:
# dmidecode 3.2
Getting SMBIOS data from sysfs.
SMBIOS 3.1.1 present.
Handle 0x001E, DMI type 16, 23 bytes
Physical Memory Array
Location: System Board Or Motherboard
Use: System Memory
Error Correction Type: Multi-bit ECC
Maximum Capacity: 64 GB
Error Information Handle: 0x0021
Number Of Devices: 2
Handle 0x001F, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x001E
Error Information Handle: 0x0022
Total Width: 72 bits
Data Width: 64 bits
Size: 4096 MB
Form Factor: SODIMM
Set: None
Locator: DIMM 0
Bank Locator: P0 CHANNEL A
Type: DDR4
Type Detail: Synchronous Unbuffered (Unregistered)
Speed: 2667 MT/s
Manufacturer: 08F7
Serial Number: D7
Asset Tag: Not Specified
Part Number: D4XS12081SH26A-C
Rank: 1
Configured Memory Speed: 2400 MT/s
Minimum Voltage: 1.2 V
Maximum Voltage: 1.2 V
Configured Voltage: 1.2 V
Handle 0x0020, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x001E
Error Information Handle: 0x0023
Total Width: 72 bits
Data Width: 64 bits
Size: 4096 MB
Form Factor: SODIMM
Set: None
Locator: DIMM 0
Bank Locator: P0 CHANNEL B
Type: DDR4
Type Detail: Synchronous Unbuffered (Unregistered)
Speed: 2667 MT/s
Manufacturer: 08F7
Serial Number: 8C
Asset Tag: Not Specified
Part Number: D4XS12081SH26A-C
Rank: 1
Configured Memory Speed: 2400 MT/s
Minimum Voltage: 1.2 V
Maximum Voltage: 1.2 V
Configured Voltage: 1.2 V
Rob@Rivendell:/$
Interesting that the max RAM is stated as 64GB.
Back to the GUI and everything looking ok and now rocking 8GB of RAM like it is 1999, again.