Companies are throwing away old hardware (like 8th/9th gen Core i5) that's perfect for running Home Assistant. See if there's an e-waste recycler near you - they might let you buy an old system for a nominal fee.

While there are some problems with used minipcs (notably drivers), i don't think they are relevant if it is only going to be used as a Home Assistant

Booting from USB drives has worked well for me

You can connect an SSD to these boards and boot from that instead of an SD card. Definitely the way to go especially considering SSDs dont really cost more than SD cards.

Always run mine in a VM along with everything else, no need for special hardware.

Get yourself a PoE Zigbee and/or Z-wave receiver and you are good to go, can even live migrate HA if you are fancy

Seconding esp devices. Especially running open source firmware (I used tasmota, there is also esphome and probably others) super reliable and much lower risk of spyware.

Both! Connecting Iot devices to your LAN is a security risk, since they sometimes carry malware. Many DDOS these days are performed by smart bulbs or the like. May even sniff in your network. Of course you can firewall them, but why bother with wifi at all then? Zigbee is pretty reliable and works even if your router goes down (you probably can't access Home Assistant Web interface anymore, but zigbee light switches etc will still trigger zigbee bulbs, zigbee thermeters will still report their sensor values etc).

I'm a Zigbee user so I use that in the examples but I'm sure the other options are fine as well.

Most wi-fi smart devices use cheap components and will often drop off the network for seemingly no reason. As long as you do some basic planning, Zigbee and Z-Wave are pretty solid. There are some folks here who will say one or both of these technologies are on their way out because Matter & Thread. I’m not sure I agree with them, but even so, HA supports Matter, so… when Thread devices become as ubiquitous as Zigbee & Z-Wave devices, you can replace them as they die.

Most WiFi devices these days are cloud-based, at least on the American market.

If you can find a Zigbee or Z-Wave (or the up-and-coming Thread), those only work locally. The only way to get remote control is to expose HomeAssistant (or a similar hub) to the internet - usually using a VPN from your remote devices into your network.

It’s a bit more upfront work, but no one can track your devices, they’re off grid/much harder to hack remotely, etc.

I did this with ESPHome on an ESP8266 connected to an IR led for some rope lights that had IR control. The hard part is finding the codes for each control, but there's a surprising amount of info out there.

Even if you build your own thing to communicate with the AC, Home Assistant is still useful since it lets you easily automate things and interact with other devices, and you get a bunch of things included (nice UI, storage of historical data, dashboards, etc). You could build your thing as a Home Assistant integration.

That might depends on how your AC is working, I recently did the same at my dad's house and run into issues because the temp control for the unit is reading from the remote. So when I sent infrared command to turn off unit, it worked as expected for some minutes then turned back on because remote sent back command as temp wasn't meet. Still need to do a lot of testing to find a way to make it work, might need to disable remote and use some temp sensors for the triggering part but not sure how that will work, might need to use some hysterisis friendly climate addon for that...

It's been a while since I messed with home automation, but ESPHome was amazing to program ESP microcontrollers (i.e. you most likely wouldn't have to write any code). You can use ESPHome devices with both Home Assistant and Openhab (using MQTT, IIRC). The last I checked, it was easier to program your own functionality in OpenHAB than Home Assistant.

I got into each mesh technology for specific devices. Home Assistant supports them all and they seem to coexist just fine in my use case.

I have a small to medium setup with only a few simple automations and a focus on voice control and scheduling

Preference

• Thread - given Apple, Amazon, and Google support and the standardization work, I expect this to be the future. Eventually. But I’m getting impatient. If I’m buying a device, I prefer Thread but usually it’s not yet
• z-Wave - my first, and most devices. Basically this was what was most available at local stores when I started. No complaints
• Zigbee - by far the biggest selection of simple, cheap sensors. I need to more of those
• all too much is WiFi but I try to avoid

But it also helps that my approach is generally switches and outlets. Hard-wired, predictable network, tend to be repeaters. I have comparatively fewer leaf nodes.

This approach also fits in with my biggest challenge. While my house is small, it’s an older one with dense materials that blocks a lot of radio signals. For example I have no cell phone reception inside yet strong signal just out any door. My focus on switches and outlets overcome this with a repeater in every room

So for example a few years back I got a z-wave IR blaster to control a mini-split AC because at the time I mostly used z-wave. I already had a z-wave light switch in the same room, acting as a repeater, so no worries about connectivity. Now I have both z-wave and Zigbee light switches in that room so expect both meshes to be strong for any future devices in that room