Two different questions.
SPI: yes, bit-banging is fine. The SX1276 has no minimum SPI clock, mode 0, and register access at a few hundred kHz from a PIC is plenty. People run raw LoRa point to point like that.
LoRaWAN: that’s where a PIC16 runs out. The stack needs crypto (AES-CMAC for the MIC), session state, and it has to open the receive windows on time after each uplink. Hermetry’s default RX1 delay is 5 s, and the window has to open within a few milliseconds of that. Doable, but not on that RAM.
So Kevin’s idea. A module with its own LoRaWAN stack (an STM32WL based one, for example) over UART. The PIC sends a join command once and then a hex payload per reading, the module handles everything else. Each probe joins with OTAA, Class A, LoRaWAN 1.0.2 to 1.0.4, which is what Hermetry’s network server does, on the same frequency plan as your gateway. A table codec turns your bytes into moisture and battery. Check LoRaWAN limits before you pick the module so its version matches, and buy it for your gateway’s band.