I would suggest NOT braiding and keeping in a tail bag in the summer if your horse has any sort of turn out. I have seen far too many incidences where the horse was swishing his tail furiously trying to rid himself of bugs, getting it caught on something (fence, tree, shelter, even in the stall) and ripping nearly an entire third of the braid out. Without that braid it is likely only a few strands would have been pulled out.
Personally I only put the tail up in a sock starting in the fall when the bugs go away, until the bugs return in the spring, then leave it down except for the night before a show. You do as you already said, wash, condition, and put up for no more than 2 weeks at a time (preferably less) but instead of using a brush or comb, use your fingers to gently comb through the tail and mane. Do NOT braid above the tail bone, start about an inch or 2 BELOW the tail bone. I don't have as much of an issue with keeping manes braided, but make sure there isn't something they can scratch on that a braid can get stuck on, I usually use a small strip of sheet in the mane instead of bare braids, it keeps strands from rubbing on each other and can hold conditioner in longer.
Had bad experiences with MTG, rubbing and hair loss when used as directed, and it STUNK horribly, left a very greasy/sticky film. And absolutely don't braid too tight or you will have a horse that gets irritated and scratches.