Welcome to the Forum!
Are you sure Mack had SBD and not constipation? They look pretty much the same. Sometimes SBD happens in Betta for no real reason.
Keeping Betta is really pretty simple: Good food, clean and warm water.
Cycling and testing:
Always test water *before* a water change. Water can look immaculate and still not be acceptable parameter-wise. If you are seeing .25 Ammonia your tank is not cycled. Here is this Forum's cycling tutorial. I hope you find it useful.
Frozen foods will store in the refrigerator for a week without spoiling. Pour a small amount in a net and rinse. Use enough to feed your boy and Finn and store the rest. Frozen can be fed daily and not just as treats. I feed only frozen with the occasional Fluval Bug Bites and NorthFin Betta Bits thrown in. My preference is to feed 2x per day until the belly is gently rounded. I do not count but some find it easier if they do.
For a 10-gallon, cycled tank, one 25% water change per week (with vacuum) is good. I test daily until my tanks are cycled, once per week for a month after to make sure they are stable. If everyone is active and eating I then test every three to six months. But, how often we test after a tank is cycled is personal preference and opinion. For those new to aquatics I would suggest once a month or six weeks until they are more confident in and comfortable with fishkeeping. And if you even *think* something is off, even if inhabitants are eating and active, test.
Having had Betta-based community tanks for 40 years, I have come to realize that many people have trouble with swimming tank mates and Betta because the tank is not properly planted and cycled. There are, of course, those Betta which do best living alone and should be left that way. That is why I always suggest one have a back-up plan.
By properly planted, I mean with enough plants that the Betta cannot be seen at a single glance. This gives the tank mates and the Betta areas of retreat. In addition, Betta are shade-loving fish; they do not do as well when lights are bright. Not to be confused with length of time lights are left on.
I do not separately feed my tanks but feed enough for everyone. But, remember, I feed a variety of foods and not just one or two. And the only herbivores I keep are Otocinclus.
I do occasionally clip a piece of a seaweed sheet to the tank for herbivores (Oto) if algae and plant detritus is sparse. As omnivores, bottom feeders such as Corydoras do not really need algae wafers, etc., if enough variety is offered. I found if I fed the same thing to everyone I didn't have trouble with the Betta pigging out on, say, algae wafers.
***Almost forgot: I use SeaChem Stability to both cycle my tanks and to maintain a stable tank when new residents add to the bioload.
Anyhow, just my thoughts. Hope you enjoy the Forum. We do our best.