I use a bunch of Xiaomi bluetooth temperature / humidity sensors all over my house. I recently found these though that I like even better:
https://smile.amazon.com/gp/product/B07WZNWWJK/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Then you need ESP's running ESPHome to listen for the Bluetooth broadcasts that forward the readings to MQTT. I'm working on some stuff that will work on a Raspberry Pi or other Linux system with a Bluetooth adapter to do the same.