I've used greenbeanroasters.com in the past. I've played around with roasting green beans using an old air pop corn maker, which is fun - interesting explorer how the flavours change as you go from light to dark. I think air fryers can work too for small batches.

As for varieties, I generally like Colombian beans for their rich chocolatey flavours.

Water temperature, extraction time, and grind size are the main variables dictating coffee taste. Aeropresses are so handy because you can easily control water temperature and extraction time. Get a burr grinder (not the blade grinder) to play with grind size. I have spent a lot of time playing with these three factors alone.

Too bitter? Just run the bottom of the pot under a tap before it starts "sputtering". This ensures the water doesn't get too hot during extraction.