Not sure if you're still researching, but I really like the Bluetooth ones.
For the Pi4, I use a little USB Bluetooth adapter from pluggable ($13-ish) on a 10-ft USB extension cable to get the module away from other things. I think the pi has Bluetooth built in? But I never got it working. But generic little Bluetooth USB things are perfect. With the USB extension cable I have great whole-house coverage (small house though)
Then for the sensors, I get a 3pk SwitchBot IP65 indoor/outdoor hygrometer.
They're on sale on Amazon right now for $25-ish for 3, so it's a pretty good deal.
As far as placement, I have some outside - they're fine. I try to place them where they're not getting totally hosed by rain or snow - under a tree or in the greenhouse or under my garage eaves sheltered from direct sunlight and rain.
They are very low power consumption (2x AAA batteries included) and I've had some running over a year and they still have more than 50% battery. (The battery level does report to home assistant)
They show up on home assistant immediately after powering up - no apps or anything needed. Just adopt them and set their location in the home assistant settings.
This is exactly how my local municipal fiber network works. The the county owns, and builds put, the fiber network and maintains it, selling network access to local ISPs who sell to customers.
Only shitty part is that if you want to have a connection built out that isn't on their plan, you have to fund the fiber run to you from wherever the nearest spot is, and that can be many thousands of dollars.
I imagine if we expanded the program like you're talking about in the rest of the world, we could actually run it fine, like, we have the ability to... It's just that the people in power are fucking awful.