Use MiniCore; it's made for applications like this. It gives you much more flexibility and is actively maintained.
The internal 8 MHz oscillator works great but is not as accurate as an external resonator/crystal.
I've not come across Minicore before, I'll have a google and see if it looks easier to me. That's both people who have mentioned it, so it's promising.
MiniCore would be the best choice!
When I mentioned Lilypad, I did not mean you should use a LilyPad board but only use the LilyPad bootlaoder with your ATmega328 microcontroller (as LilyPad does also use a 328 but with the internal clock source of 8 MHz).
The LilyPad bootloader is part of the Arduino IDE.
The bootloader (fuse settings) will not change any voltage.
There are two different versions of Arduino Pro Mini:
Arduino Pro Mini 328 - 5V/16MHz
Arduino Pro Mini 328 - 3.3V/8MHz
They have different voltage ragulators.
If you want to make your own "stand alone" board with ATmega328 take a look here:
Gammon Forum : Electronics : Microprocessors : How to make an Arduino-compatible minimal board
I understood about the Lilypad bootloader, I was just getting at the fact I know nothing about that board. I guess it makes no difference really, as it is the same chip.
Currently I'm using a Pro Mini 3.3V as it's being used with a BME280 sensor and a nrf24l01+. I thought I'd have a go at creating my first PCB but as it stands, I thought it'd be too basic to use the Pro Mini. Which is why I'm going to try and get a stand alone chip to work. I wanted to stick with the Pro Mini bootloader, as the brown out works well along with the two AA batteries, which are powering the circuit.
Now I'm thinking I'll need a voltage regulator to maintain 3.3V, for all the other modules. In my mind I was still going to run those from the chip, stupidly.
Any reccomendations with regards to the volt reg? Maintain a 3.3V to power the chip and all of the modules, or just power the chip directly from the battery source and just have a volt reg to power the rf24 and bme separately. Again, if I'm making sense there. thanks