I have a couple of dht22/d1mini (esp8266) devices for two rooms…These are my important sensors for controlling the heating.
But for all the other rooms I have the xiaomi humidity and temperature sensor. I do have their aqara gateway, but have migrated all the temp sensors off and on to zigbee2mqtt and CC2531 coordinator and routers. Might be an option for you.
I have several Z-Wave sensors from Zooz that report humidity, luminance, temp, and motion. I also use NodeMCUs with various sensors and MQTT. These are are my preferred sensors because they cost almost nothing, and I can setup whatever hardware I want with them and write the code myself. I originally set them up with 18650s, but they are so small that I just plug them into a 5v USB power adapter and leave them at the outlet.
Here is the 3 best ratio quality/price :
Among 3, I would prefer the first for indoor as a simple ESP32 can read it and it is very inexpensive.
The 3 can be used with OpenMQTTGateway. Here is a list of all the temperature sensors compatible with OMG
These are some neat looking sensors!.. Why build your own, when you can have these… I like.
As you are looking for cheap sensors and mqtt integration, this is what I am using here.
I use the Xiaomi/Aqara Temp/Humidity/Pressure-Sensor and Zigbee2mqtt.
The sensors are very small and the battery lasts for more than one year (at least for me)
Maybe worth a look.
Thanks for the suggestion, I doubt that cheap Xiaomi stuff are very reliable and accurate but for that price i might get a few extra just in case. I also checked and it seems that flashing can be done quickly and OTA. The issue is that this requires to build a OpenMQTTGateway or multiple ones to cover 300m2 house over 3 floors. I will look deeper into this
which hardware are you using for Zigbee2mqtt ? is coverage good for multiple sensors ?
I began by building my own sensors indoor and outdoor, I learned a lot, but it took me so much time…
Now I have less time, I want things that work, that looks nice, and there is more device offering than 4 years ago.
So I don’t mess up any more building sensors for my home 
The only issue that I got with a Xiaomi sensor is when I forgot one outside in the rain. Outside this, I didn’t get any other issues with them.
What accuracy are you expecting?
A few central gateways with external antennas EA could be enough (ESP32-GATEWAY - Open Source Hardware Board)
I’ve had great luck with the Shelly H&T using MQTT to keep it all local.
I’ve three DIY NodeMCU running ESPEasy with DHT22s and photoresistors. I’m not running them off battery and you are not looking for DIY so I won’t same more about those.
I’ve also three Govee H5072. They run on 3 AAA batteries and I’ve had one going since March and it’s at 80% battery level. However, it’s BTLE and there is no binding for it so I had to write (with the help of a couple of other projects) an addon to my sensor_reporter that listens for the broadcasts from these sensors and publishes them over MQTT.
I’m very happy with them. They run around $15 each which is a little high but it has a screen (which was also a requirement for a couple of my sensors) and most of the cheaper ones do not. But they have a very fast reporting time (every second). And I’m already running sensor_reporter on a lot of machines so it’s no big deal to spin up a special service for this purpose.
I do not regret either choice.
The one I use is this: https://www.tindie.com/products/slaesh/cc2652-zigbee-coordinator-or-openthread-router/ running on my Raspberry PI 4.
But you can choose from many supported adapters: https://www.zigbee2mqtt.io/information/supported_adapters.html
Colleages of me are running Conbee´s and are happy with it, too.
I won´t recommend the CC2531, for me it was not very reliable when using Zigbee 3.0 Support.
The coverage depends on your Zigbee network. You can have some Zigbee-Bulbs that act as a zigbee-router/repeater without any additional hardware, or use one zigbee2mqtt instance per floor reporting to your central mqtt-broker.
I used the original Xiaomi-Gateway before, and it was very reliable and could run without cloud. But when Xiaomi brought out their Zigbee 3.0 Luminance Senors they were not supported on this gateway.
I switched to zigbee2mqtt and never regret it.
And about the accuracy:
That is from the data-sheet, but for the temperature I think I can say it is in this range.
Kindest regards,
Christian…
What is your experience with battery-life of the Shelly H&T?
I never was able to get good results with batterypowered wifi devices 
14 months and going strong. They advertise 16 months per battery.
I finished writing a bluetooth binding for all of Govee temperature sensors a few weeks ago. I’m using a h5074, they are great little devices. Currently waiting on the PR for the binding to get merged but I expect it will get merged soon.
That’s great! The only problem for me is I don’t have BT on the machine where OH runs and I’m out of usb ports. I’ll have to get a hub and a single to use it. But having a binding is fantastic! They are great devices!
Another vote for Xiaomi sensors
I use them everywhere. Cheap, long lasting battery, accurate and very small.
Matched with the conbee controller and nodered dashboard for logging.
I have them in every room and they decided if the thermostat turns on or off depending on temp.
Since I started this topic over a year ago, I just wanted to say a big thank you to you all for the various replies with a special thanks to @1technophile (also for the impressive work he has done on openmqttgateway) Merci !
I ended up getting the excellent 19€ Olimex ESP-32 gateway board and small Xiaomi Mi Temperature and Humidity (LYWSD03MMC version 2) which I could very easily flash OTA with the flawless ATC firmware
These are cheap 4€ devices that I found on Banggood. It was cheaper to get then per pack of 4, also there are shipped with the CR2032 battery wich honestly last a good 9 months.
I took 20 of them and throw one in each room !
Accuracy is not the best (as expected) but I found a more accurate T/H source s reference and it was possible to calibrate them by manipulating the offset. At the end this solution is good enough for general household T/H monitoring.
MQTTGateway is configured to listen to BLE and in no time I have added them all in OH3 with the MQTT binding + mosquito server.
Config was super easy, when you have one it is just a of copy paste effort.
It turned out that coverage for a very large house is OK with just one gateway. I placed it in a central location and it i can pull from 4 floors including basement. I took the ESP32 with external antenna and the matching metal case (8€) . To my surprise its able to get signal all the way from the garage and even outside (just behind a windows though)
MQTTGateway has also helped me to integrate more Mqtt devices (Tasmota/Sonoff such as TH16 and POWR2 for the hot tub heating control) as well as a plant monitoring sensor (Mii Flora)
The fact that it comes with a LCD screen, allow a quick look at values when passing-by but most importantly data is collected in the background and kept into OpenHAB persistance.
Next year I am changing my central heating system and I wanted to have historical values in order to compare them when the new heating is installed.
20 sensors for less than 110€. I found that this is the most “cost effective off-the-shelf solution”, looks good, high WAF, no soldering, cloudless and easy to setup.
Thanks again
Just to add another option: $25 for the sensor and $25 for 433Mhz receiver. Howto is here. Configuration is pretty easy. With Ubuntu no need to compile anything… Everything is available through apt.