4 GB RAM limit

[Arwen]

One odd thing I have noted is that ARM 64 Linux distros seem to use less memory than a AMD 64 type. It's possible that the ARM 64 instruction set is simply more compact that AMD 64.

In the past, you have been running the default Debian with its 32 bit userland. Are you comparing by any chance a 32 bit ARM userland with a 64 bit x86 userland?

Yes, that may account for it. One thing to note, ARM 64 bit still uses 32 bit instruction sizes, (at least according to Wikipedia).
I was unable to find the size of x64 instructions.

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Aside from the RK35xx series that are still coming out, the current upgraded "RK3399 Pro" SoC has existing products with higher amounts of RAM. For example: https://www.seeedstudio.com/ROCK-PI-N10-...-4381.html

So yeah, more memory and a replacement mainboard in the future with a different SoC is feasible, if not part of the roadmap.

If you read the description of the Rock-PI device more carefully, it separates out 4GB for CPU and 4GB for NPU.
It's not clear if that can be changed.

I did find another RK3399 Pro board with 3GB / 6GB for memory. But, again, it's not clear, though it does not mention a separation for the CPU verses NPU.