Hi! I have previously made a Tx device that sends and Rf signal containing a code to an rx device, it was made from an arduino pro mino 3.3v, a voltage regulator module and a LoRa Sx1278 Ra-02 RF module. The space occupied is quite big and I wanted to make a custom PCB. For the PCB I would use an Attiny84 SSU, and an Sx1278 Ra-02 chip, without the breakout board. The tx device send a signal containing a 4 digit code at the press of a button, which usually happens at 20-40minutes intervals. Would the Attiny84 work for this project? I chosed it because it's easy to solder and cheap.
- Why not prototype it to find out, that’s what everyone else does ?
I was referring if the current LoRa library made by Sandeep Mistry is compatible with the Attiny mcus, and if the software SPI of the Attiny would work with the LoRa module. If someone knows and can tell me that would prevent a loss of resources.
You could check whether the basic LoRa library examples will compile and fit into the ATtiny84 memory.
If you are going to do a custom PCB you could consider the Attiny series MCU's. For example: attiny3214, attiny3224. They have 14 pins, 32KB, hardware SPI, can be programmed with just one pin + GND (one-wire UPDI), what is very convenient. And are small, very low power and cheap.
The package is SOIC, but for prototyping in the breadboard you can use an adapter board.
I haven't tried LoRa yet, but I see some people doing things with these MCU's GitHub - Hudson-Sonoma/GBT1: Seeed Grove Base for Microchip ATtiny3224 <- uses a basic lorawan library for AT commands.
Or maybe you will need to adapt the libraries. You could try to setup a project and see how far is it from compiling 
Thank you, that's a good ideea, I cannot seem to find the Attiny3224 easily, but I can aquire the Attiny1614 easily enough, that would work, right?
Yes, it's the same than the attiny3214 but with 16KB. But be careful with the fakes in Aliexpress.
As they are not from Chinese companies it's easier (and safer) to find them in digikey, mouser, etc. and cheaper, except for the shipping cost.
I don't know if LoRa will work so easily with the same libraries, probably you will have more work/fun
maybe simpler to buy a PCB with onboard microcontroller and LoRa modules, e.g. HELTEC® LoRa 32, Lilygo LoRa32 V2.1_1.6, etc