I’m currently using a lot of those mini hygrometer sold under a lot of brands. Mine are branded Brifit, but I’ve seen other brands like Oria and Ankilo and many others.

Here is a listing from Amazon, please don’t buy it there, but to get an idea of the price and the specs:

www.amazon.de/dp/B0D1FS8VR6

They use bluetooth and are powered by a little CR2477 coin cell battery, which lasts about a year (varies a bit, I’ve gotten some to last 10 months, others 14 months). I bought a bunch of them at once, which drops the price a lot. I’ve been using them for about 2 years now and they seem to be accurate and report data often (every few seconds).

My Homeassistant server (tiny old Intel Nuc thing) is pretty central in my home and I have a USB bluetooth stick attached to it. It’s on an USB extension cable to have the antenna of the bluetooth stick out of the enclosure it’s in for better reception. The sensors are scattered throughout the house and all seem to have an excellent connection. The USB stick I use is a UGreen one which is very common, with excellent support in Homeassistant.

I think this is the stick I have, at least the picture matches, again please don’t buy from Amazon.

www.amazon.de/dp/B0BXF13GB7

UGreen stuff is pretty good and sold in a lot of places.

Both the bluetooth stick and the sensors are China special, but these days it’s very hard to find anything that isn’t. Quality seems great tho.