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 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.
[0] https://github.com/maribu/at86rf215-breakout