Kevin’s right that HC-12 is the easy way. With bare modules, the piece most people miss is that the Si446x chips don’t have a register map you just poke. You load a configuration blob that Silicon Labs‘ WDS tool generates (radio_config.h) for your frequency, modulation and data rate, and send it to the chip at startup.

The library that works for me is RadioHead, driver RH_RF24, which supports Si4460/4461/4463. It ships with a set of ready configurations, so you don’t need WDS unless you want something unusual. Minimal sender:

#include <SPI.h>
#include <RH_RF24.h>
RH_RF24 rf24(10, 2, 9);   // CS, nIRQ, SDN
void setup() {
  Serial.begin(9600);
  if (!rf24.init()) Serial.println("init failed");
  rf24.setFrequency(433.92);
  rf24.setTxPower(0x4f);
}
void loop() {
  uint8_t msg[] = "lvl=182";
  rf24.send(msg, sizeof(msg));
  rf24.waitPacketSent();
  delay(60000);
}

Two traps. SDN is active high, so if you leave it floating the chip sits in shutdown and init fails. And these modules run at 3.3V: a 5V Pro Mini needs level shifting on SPI, or use the 3.3V/8 MHz Pro Mini, which is also better for battery life.

On the Pi end, read the packet and publish lvl as a key the same way you do for your other devices.