@andrew_cb
Humm.
Nice write up.
Most of what you have written exsietd already in my head, as I used these kind of questions when I had decide what type of device to buy, or to be more exact : what storage options ?
pfSense, when used without any addition - read : packages - runs just fine on any device, from the 1100 to he 8x00, and the only difference will be ! how many NICs, and the overall throughput.
Anyway, for the reasons you've shown, I bought a 4100, and I had the option : base or max, I went for the max, and you've summed up the real (for me) reasons. Which tells me that the mmc can be replaced (added ?) with a NVME drive rather easily on that (4100) device.
Still, I'm not using the packages that create huge amounts of write cycles.
I do use pfBlockerng with a minimal DNSBL list.
And you're right, when we go out buying that no-name router, we don't look for "what RAM" or what storage it has. But pfSense brings new issues : suddenly, there are a lot of things to click and activate.
And these new functionalities often start to 'do' things on the router (pfSense) and they log a lot so nice charts and images can be shown (and pfSense isn't even a pi-hole yet).
edit :
We all need to install and use pfSense for a while to discover what device (hardware) we actually need.
And while using pfSense, our needs change.