In an earlier episode I asked for Plugs with Wifi and ended up with Gosund and Tasmota. Considering this to be a success, I asked the Twitters for Temperature and Humidity Sensors. Of course, this got complicated.
Christian Kahlo mentioned GY-BME280 Sensors, which can be connected to any ESP Tasmota device, and work with the “sensors”-Edition of Tasmota just fine. Worth a try. Jens Dibbern points to generic NodeMCU modules and BME Sensors again.
Wickelkranz points at the Aranet 4, but 150 Euro is too much to play with it randomly. This would need more pre-buy research. The FAQ is not encouraging, but the Forums are helpful and point to Github.
Sascha Köhler recommends the Wemos D1 Mini Development Board and then a DIY sensor build.