Help a newbie understand the possibilities of Home Assistant

I know I am keeping on insisting on something you already explained to me, but I need to ask. Can I use a primary, smaller media for the OS and a bigger external storage for data logging, if that is the primary use of space? If so, how many space would be needed for the OS install alone?

Also, would I be wrong in assuming the great majority of writing cycles happen for logs, while the underlying system remains mostly untouched? Because in that case I could use an SD for the OS, and an SSD to keep elaborated data.

Lastly, if I am to use the RPi3 I already have lying around, I won’t have USB3.0 available. Will the USB2.0 ports I have be too slow? Do I have to upgrade my system for that reason alone?

Depends on your install choice. If you use HAOS you have limited options, if you use Container you can do whatever you want.

An SSD will still work, you’re just going to have limited performance, but that’s a statement that applies to everything to do with the Pi3.

Would you be kind and further expand on the make/models? I am searching for a cheap (wifi/Zigbee/Z-Wave) elusive wall switches that work directly with HA. Did I mention cheap? I also cannot have even initial setup connecting to the internet as I have no internet (or cell) in this location.

I like WiFi- it just works. I recently bought some Kuled switches because they come pre-flashed with Tasmota. (I don’t use Tasmota much and the first thing I do is to put ESPHome on it).

I don’t think you will find a WiFi switch that does dimming or color changes. For that you need Z-Wave or Zigbee, but those devices cost at least double the price of WiFi.

If you need three-way switching, Z-wave switches will do it. I don’t know about Zigbee. (FWIW, in three years I have had two z-wave switches die- but my WiFi switches just keep working).

Dimmers ain’t a problem (look here under dimmers)
But color, yeah, that is a problem with WiFi