Nice
. Welcome to the world of making smoke out of green coffee beans.
You will have to learn how to listen for the sound of First Crack, and then how much longer to roast after that. After that comes a gap of variable time length (depends on how much green beans, their type, and your roaster) followed by a different sounding Second Crack.
The only way to learn what roast level I liked was to time things after First Crack started, and by roasting longer & longer by 30 second intervals, eventually roast darker than I liked. (I stop well before Second Crack.) It will take some time to learn this.
I strongly recommend a small digital food scale to weigh out green beans. These electric roasters can only heat up a limited amount of beans. Darker roasts may require smaller starting weights. Keep track of those starting weights until you have a feel for what your roaster can handle at the roast level you like.
My favorite source of green coffee is Sweet Maria's
https://www.sweetmarias.com/. They have consistently provided the best selection of different varieties of coffee, tested & re-tested for freshness, and their opinions about the various coffees have been useful for me.
The downside is they are in California, and shipping to the east coast costs a flat $9 for up to 20 lbs. As a result, I make larger orders. But it took me a few years to really know what I like before I could confidently order larger amounts.
I like beans from Guatemala, Ethiopia, Kenya, Panama, and Costa Rica. Because you like espresso, pay attention to what is said about espresso and roast levels for each type of bean, for example
https://www.sweetmarias.com/product/ethiopia-kaffa-michiti-coop-4774.
Also SM's lists all green coffees in stock by whether they think they are good for espresso
https://www.sweetmarias.com/category/good-for-espresso.
Right now is not quite a good time to buy green coffee. Most of what is now for sale is last year's crop. Anytime in the next month lots of new crop Central American beans should be available. Wait for them. East African coffee is on a similar schedule.
You might like coffee from Java and Sumatra. If you like those Indonesians, learn how dry processed and wet processed coffee beans taste. There is a big difference. I strongly prefer wet processed.
Sweet Maria's sells samplers that might be useful for you to try.
They also sell blends they say are good for espresso and dark roasts. You might try some to get a feel for what they think good espresso is and compare that to what you like.
https://www.sweetmarias.com/category/green-coffee/others/blends#