You clearly know more than me, but wouldn’t everything from 4GB to 1TB have the same number of walks? And one more walk gets you up to 256TB?

No that’s not how it works. Handling a larger address space (e.g., 32-bit vs 64-bit) maybe could affect speed between same sized modules on a very old CPU but I’m not sure that’s even the case by any noticeable margin.

The RA in RAM stands for random access; there is no seeking necessary.

Technically at a very low level size probably affects speed, but not to any degree you’d notice. RAM speed is actually positively correlated with size, but that’s more because newer memory modules are both generally both bigger and faster.

The easiest way that doesn’t affect the main network would be to use a travel router. Its WAN IP would be the private IP it gets from the main network (over wireless since that’s your only option). And it would NAT your network onto that IP and then you can do whatever you want on your network.

I’m not sure if that Mikrotik router will do this but it might. You basically need something that can connect to an SSID and use that interface as its WAN interface. The wireless factor here is really limiting your choices. If you had a wired uplink to the main network you could use any router/gateway/firewall you wanted. You could also use an AP in bridge mode to connect to the main network’s SSID and wire it to the WAN port of any router of your choice.

You don’t really need to use VLANs to separate your network from the main network unless you want to share any of the same layer 2 segments (basically wired Ethernet) while keeping it isolated. But it doesn’t really sound like that applies in your scenario. Of course using VLANs within your network would still make sense if that applies (for example, to separate your server traffic from your IoT traffic).