I feel like your best bet is ESPHome and the ESP family of devices. Last time I checked you could get the parts for a project like this off Amazon (with spare parts) for under 20$.

https://esphome.io/

Wouldn't any off the shelf temperature+humidity sensor work? I use Sonoff and Aqara ones with Sonoff Zigbee coordinator and Zigbee2MQTT, both are somewhere around 15€ a pop, integrate painlessly and do the job. Aqara is a bit smaller and has air pressure sensor, too.

If you have a Bluetooth proxy setup you can get Switchbot devices pretty cheap. They have an outdoor temp/humidity sensor for ~$15. I bought a pack of 3 for $36.

Bluetooth proxy can easily be setup with esphome and a ~$10 esp32

Not sure what temp and humidity levels a terrarium is expected to have, but Shelly makes a good device for monitoring both - https://www.shelly.com/en/products/shop/shelly-h-and-t-white.
Or one with a screen (still smart) - https://www.shelly.com/en-us/products/shop/shelly-h-and-t-gen3-1/
We use the latter in our bedroom which triggers the power outlet to a humidifier if it gets too dry when we're sick.

I have some sensors on a hognose enclosure and I can say for sure stay away from INKBIRD stuff. It integrates poorly and fails on loss of internet connection.

I don’t have a good solution for the heatmats yet but I’m looking into integrating an aquarium controller I have laying around to manage that.

For hygrometers there are a ton, but you’re going to want to look for low power and probably zigbee. the majority of the sensors I’ve tried have been an annoyance to manage batteries but the zigbee ones seem better.