Most efficient & easy would probably be a nrf52840 (you can get the nrf52840dongle for $9, it has a wide voltage range and low power sleep modes and USB).
You can communicate over the 802.15.4 radio which is very low power compared to WiFi.
I'd use RIOT-OS, Zephyr has official support from Nordic, but that's what I'm familiar with: You can create a 6loWPAN network where each node gets an IPv6 address, they can communicate to the IPv6 world through a border router which would be another nRF52 Dongle where the USB serves as an USB-Ethernet Uplink that you plug into your router.
You could then use CoAP to send the data to a server somewhere (or just plain UDP) and sleep most of the time.
There are single chip solutions like the SAM R30 but `samr30-xpro` is a bit more on the pricy side and I'm not aware of any cheap boards with the MCU.
You can use SPI Radios though (if you don't want to design a board yourself). CC1101 isn't IEEE 802.15.4 but if you don't want to talk to third party modules that shouldn't matter.
AT86RF215 is a dual-band chip with pretty good range, but besides the official (expensive) eval board, there is no ready-to-buy board. You could manufacture them yourself though. [0]
Of course there is also LoRa which would likely be enough for your use case. It just has a much lower frame size.
You can communicate over the 802.15.4 radio which is very low power compared to WiFi.
I'd use RIOT-OS, Zephyr has official support from Nordic, but that's what I'm familiar with: You can create a 6loWPAN network where each node gets an IPv6 address, they can communicate to the IPv6 world through a border router which would be another nRF52 Dongle where the USB serves as an USB-Ethernet Uplink that you plug into your router.
You could then use CoAP to send the data to a server somewhere (or just plain UDP) and sleep most of the time.