I too started out on the breadmaker / heatgun approach, used it for 2-3 years for roasting coffee (and get my beans from greanbeanhouse too). It really is fun, messy but fun. You don't need to modify the breadmaker so long as it has a good "dough only" mode. Mine was a Breville somethingorother that had a dough cycle that ran for 30+ minutes, ample time as most roasts were around 15 mins +/- depending on conditions. I also built a wooden stand to go over my breadmaker that I mounted the heatgun too, this meant a consistent height above the beans and you don't have to stand there and hold the HG for 30 mins. If you search on www.coffeesnobs.com.au for a "coretto", this was one of the originators of the method.
A few years ago I upgraded to a Behmor1600plus home roaster. The difference in the quality of the roast was immediately obvious. I considered myself a fairly experienced home roaster using the BM/HG combo, but the taste profile of the beans fresh (rested 3-7 days) from the Behmor was amazing and somewhat unexpected. Batch sizes are slightly smaller than BM, 400gms in Behmor vs 5-600 in the BM, but its still plenty. Plus, I now roast indoors (on the kitchen elements under the vent), something you definitely could (should) not do with the BM / HG combo.