@alexsh1 Thanks for the comment, there is some good tips there! I'll try re-soldering some of the connections. Funnily enough, I had wanted to increase the transmission power on 2 of my nodes (the furthest away from the controller) including this one here, and had added that at the top of my sketch. After uploading the sketch, the serial monitor still displayed a lot of garbage, but the node started successfully transmitting temperature and humidity values!
So I've put the batteries back in and hung it up in the room, and it has been running pretty good so far. But I have my doubts how long it will last, given I didn't actually fix the serial output issue...
On another note, is there anyway to confirm that the transmission power has been increased? I added
#define MY_RF24_PA_LEVEL RF24_PA_MAX
at the top of my sketch, just after defining the radio type, but it doesn't seem to have improved my transmission success rate (I realise there are lot of other factors affecting transmission success, but just wanted to check)...