I've never had a problem with Dropsy but I see it a lot on here. I believe (From what I've read) that its more of a symptom of a different problem (sort of like a fever)thats usually a sign of poor conditions or a bacteria of sorts(?) I've gone full panic mode before when a betta bloated because I was so scared of him pineconing but he was just constipated and I dealt with the issue. I have nightmares about Dropsy. I may be lucky, or because I have a lot of time on my hands. I suggest, if you choose to get another betta, to regulate everything a bit more.
-Make sure the water conditions (PH, temp) are proper for housing a betta.
-Make sure he/she isn't stressed. Messing around with the tank more than you should or constant cleanings and excessive water changes can do that.
-Proper regulated diet. Bettas will eat themselves sick (literally) so be cautious not to over feed.
I go by those points and haven't lost a betta to dropsy yet.
Sounds like you've had some bad luck, =( I wish you the best.