i like both the argon and the simple heatsink setups, either work great. i did end up adding an additional heatsink to the argon, the flat case does not provide great heat exchange in an enclosed space.

you can do passive cooling as well, just all depends on how hot the location gets.

I have had some pretty obnoxious recurring reboots on my Pi4 when I don't add a heat sink and fan.

Most rpi heat sinks have a foam insulator acting as a sticky tape. Remove it and use thermal paste

I've been using this after, like you said, 2 weeks of full-time silent 3V fan, leading to a sudden increase in noise and worse performance (it essentially broke), and... damn, it cools even better now, it's more temperature-stable, and it's absolutely silent, while no extra power consumption is needed.

It is also true that I try to keep HAOS with as little concurrent addons as possible.

The case is really cool once set up, I love how it feels. It's got a purpose and it's going for it. Really feels like it was made 1000% for dissipating heat. And it works like magic.

I highly doubt homeassistant will ever abuse the pi4 CPU enough to make active cooling necessary. As long as you have a heatsink of some form on the SoC you'll likely be perfectly fine. Obviously monitor it for core temps once installed.

Remember the Pi uses the entire PCB copper ground plane in the board as a heatsink too.

Why don't you try a cpu stress test to meausure the cpu temperature with and without the fan ? I think you should try if you're quite concerned about this.

Argon case comes with software which controls fan speed and turns it off for temperatures below +50°. My RasPi with HA never went above +45°, the fan inside never turned on. I'm using it for two years now.

I used to run a Raspberry Pi 4 in the same case for months at a time and didn't have issues. I'd say you are fine