I've learned a lot about roasting in the 6 or 7 years I've been home roasting, especially in the last 5 or so years when I've had a real one pound commercial sample roaster. One thing I've learned during this process is that whatever you learned on one type of roaster probably has little ability to be transferred to another one. Another thing I've learned is that unless you have the funds to purchase a "dream" commercial roaster, it is unlikely that you will be able to obtain consistent and excellent results with anything that a home roaster is likely to purchase, unless whatever it is that you purchased has been substantially modified.
In order to teach yourself how to roast, you are going to have to expend considerable energy, considerable time, and at least some money. This is not to say that you can't produce acceptable results without a lot of energy or time, however if you want to achieve results that come close to what a really good commercial roaster can achieve, then time and effort and expense are all necessary. If all you want to do is to produce decent results that are better than what most people can easily buy locally, most anyone can learn how to do that with a popcorn popper or a very cheap air roaster.
Let's assume for the moment that you want to produce really good to outstanding results. If that is the case, there is no roasting device out there for less than a few thousand dollars that will produce excellent results without at least some modifications and effort on your part. The modifications and efforts you will need to expend will differ by the roaster, although no matter what you have it will be necessary to get feedback from people whose taste you respect, or you will be left with your own judgment which can be limiting (unless you are really truly experienced with evaluating coffees, in which case you would not even be reading this post or this thread in the first place).
Herewith, below, is a list of things I have learned about roasting that might be useful and perhaps transferable across various roasting devices.
(1) You can't shine sh*t. By this I mean, garbage in, garbage out. If you don't buy good beans you will not get good results, period.
(2) You need to be able to monitor your roast temperatures in a repeatable way, with the monitoring giving you some sort of information that you can act upon. Knowing what is going on with your temperature monitoring device when you have no way of modifying said temperature will be of no benefit, so in addition to having a way to measure temperature, you will need to have a way to modify heat input so that what knowledge you are gaining of the "on the fly" temperatures can be effected by something you can do with the heat source or ventilation or both.
(3) Don't introduce beans into a roasting apparatus that is already too hot or you may damage your beans before you have even roasted them. I can't be any more specific than this if my suggestion is to be useful for a wide variety of different roasting devices.
(4) A drying phase is necessary, but if it is dragged out too long you will end up with a dull lifeless "baked" roast. For me, this means about 3 minutes initially at a lower heat input until about 300F is reached on my bean probe, however I would hesitate to recommend any particular length of time or temperature that would work across multiple roasting devices.
(5) Getting to the onset of first crack after the drying phase must be done in a "reasonable" period of time. If you go too slow, you will "bake" your beans, but if you go too fast you will burn them.
(6) the time interval between the onset of first crack and the end of the roast is really really really important. I try to get around 4.5 to 5 minutes in during this period, during my (typically) 14-15 minute roast batches, which seldom go as far as to initiate 2nd crack. Jim Schulman and I did a very interesting little experiment a couple of years ago with some beans I'd intentionally roasted to have an interval of either 2.5 or 4+ minutes in between the onset of 1st and the end of the roast, with the same beans and taken to the same final temperature. Unlike most simultaneous blind tasting experiments we have done, the espresso shots made from the coffee with the shorter interval were GROSSLY inferior to those which had the longer interval. On every single blinded pair of shots we compared, it was very very easy to pick a "winner" and a "loser;" the loser in every instance was the coffee that had only 2.5 minutes of interval in between the onset of 1st and the end of the roast. We terminated the experiment after only about 5 shot pairs each, since we could distinguish them every time and the differences were so obvious.
(7) Rapid cooling after the roast is a good thing. My roaster cools the beans down to close to room temperature after 2 minutes outside of the roaster. I don't know what the "magic" number is as far as how long it can take you to cool down the beans, but longer is clearly not better.
(8) Whatever you do and however you do it, we are talking about one process, from bean selection to roasting to drink production. I like to make single varietal espresso shots using what is regarded by many here as a low PF dose of coffee, e.g. around 14g. Everything I do from start to finish is done with this in mind. If on the other hand I wanted to make shots from blends using 20g in the double basket, my bean selection would be different as would the way that I roast. Likewise, if I buy blends from our friends the marquee roasters, excellent and well regarded pros, and if I take their blends designed to be dosed at 22g and instead I dose them at 14g, the espresso shots taste like dishwater. Everything is connected. You have to decide what it is that you want to produce, in the cup, before you can really choose the coffees you want to roast and how you want to roast them.
What, me worry?
Alfred E. Neuman, 1955