This is a very generic question, the answers mainly depend on your existing skills, and if you desire to progress into developments that might be commercially relevant. Depending how far you want to plan your learning ahead, you might want to start simple, and upgrade the architecture/implementation as you go along.
An SBC (Pi or similar) is great if your want to focus on high level software. This question addresses some of the reasons that working with an SBC and an MCU present different experiences. An SBC is rather power hungry and might not come with built in short range connectivity.
There are small mcu boards with either WiFi or BLE built in, these are much better suited for battery powered operation. MCU boards can be coded in python, the micro:bit has bluetooth and supports micropython (but might not be well optimised for ultra-low power if you use it like this).
If you care about making a secure platform (now or in the future) then you might also care about having secure on-chip memory, good entropy sources, etc.
The 'many nodes, one hub/gateway' approach is good for battery powered devices. You can have an array of battery powered/short range devices (mesh or otherwise), communicating with a central SBC device. The SBC handles your WAN/cloud interface and also some stand-alone features if necessary.
If you make each node a peer (with wifi/WAN access) then you only need to write one software stack, but it's more complex, and you end up being reliant on LAN for any communications - so power outage operation isn't possible.
To clarify the types of devices:
SBC Single Board Computer, the Raspberry Pi is the most common. These can run linux, and might also be a NAS, a WiFi router, smart home device or a mobile phone/tablet which is running a bit of software to handle the automation task.
MCU A much wider class of device, not necessarily significantly lower in processing capability, but more likely to run a real-time OS, and more likely to use event driven programming. There are lots of small eval boards, WiFi or Bluetooth modules with some spare cycles, and dedicated small form factor boards like the teensy series. An MCU might be better to interface to certain types of sensor, and will often have a wide range of interfaces available (even displays).