The Radxa is a better value if pure performance is the only criteria.
But the real value of the Raspberry Pi products is the ecosystem and code support. It will be much easier to find tutorials and software support for the Pi Zero 2 W than the Radxa and the Pi will be supported for a long time after the Radxa has been discontinued.
Also in availability. I just clicked buy and will have a Raspberry Pi Zero 2 W shipping to me in two days. I looked on seeedstudio for Radxa products; literally nothing is in stock.
As for long-range antennas, you can check out Yagi's or Cantenna, also very nice DIY projects. The longest range can be achieved by dishes. Though check your countries dBi limit that is allowed to be emitted in any direction and then reduce your tx power to stay below that. The directivity will still help reducing interference from other directions. I like the APA-M05 for its compactness.
For the cooling, I use DHT22s and simply run a bunch of fans via a PWM-driven MOSFET, directly from the Pi, roughly following the video: https://www.youtube.com/watch?v=oJ32CMxliCQ
I send notifications (actually, not SMS) via qpush.me.
Spec of the Radxa Zero ($15):
Spec of the RPi Zero 2 ($15): The Radxa seems to be better value. I'm looking for a backup device for my RPi Zero W v1.3. The Zero has been such a amazing companion for me. I use it as Wi-Fi repeater on campsites such that we can all have internet simultaneously. Over the years I added temperature sensors, light sensors and an entire cooling system for the photovoltaic system. It now keeps track of the ambient temperature in the tent and fridge and sends me an SMS if the gets too high. Endless fun.[0] https://forum.radxa.com/t/introduce-the-radxa-zero/6550
[1] https://www.youtube.com/watch?v=huAKEbyPcBc