My mare's mane is still a work in progress as I've only been in charge of it for under a year, but this is it in April of this year:
And this is it now (Sorry it's so messy as I don't have the best pictures of it down...it's almost never out of braids, lol):
I keep it braided like this:
24/6. I take it down for a day once a week. I recommend 5-7 braids depending on the length of the neck. The reason for that amount is very functional- because it's the lowest amount that's reasonable to have. Too many braids are annoying to keep redoing all the time and I also have felt that they don't offer the same protection as a big thick braid. However, too few braids struggle to hold the hair properly and are just too big to wear every day.
I hose it down whenever it seems to have collected a bit of dirt and scrub it thoroughly at the base with Micro Tek shampoo (to keep itching at bay) and condition it with a Cowboy Magic or Eqyss conditioner, leaving the conditioner in for at least a few minutes before rinsing extremely thoroughly. Allowing any shampoo or conditioner to be left on at the base of the main can cause a lot of irritation and itching.
I let it dry naturally- it's important not to mess with it while it's wet because it's much easier to pull out and break while wet. Once it's dry I apply a thin layer of MTG to the base and spray Santa Fe conditioner+detangler throughout it and rub in a tiny dab of olive oil (as I'm rubbing in the Santa Fe and MTG), brush it out, and re-braid.
I've just also started a strict rule for myself of only picking it out by hand rather than brushing, which is very time consuming and takes a lot of patience to do properly, but very rewarding in the end. It is realistic to do, though, since it's only brushed once a week. However, if that's not realistic for you, I've gotten her mane as far as it is now brushing it normally with detangler, bottom to top.
The purpose of the braids is both to prevent tangling and wind knots and to protect the hair from other normal damage. In all of my horses' manes and tails I've noticed a tremendous difference in fullness and length when I kept up with regular hosing out and braiding of their manes and tails. As you showed in your picture, it is important to begin the braids loosely at the top to keep it from being pulled out when she lowers her head to eat and to make them more comfortable to leave in. It is also important to make it tighter when you pass the first few rounds of the braid, to keep it secure and to keep from having any big gaps that things can easily get caught in.
It's also important to only braid when the hair is completely dry, because braiding while wet prevents the hair from properly drying and can cause fungus or irritation.
I think that covers about all I can think of that I personally do. Good luck!
ETA: I would personally strongly advise against
the use of any sort of hood or covering. I've both heard awful stories and seen it proven time and time again that leaving anything covering the mane for any extended period of time only rubs it and in the long term can cause very serious irreparable damage. It will grow thinner and thinner the longer this is kept up and literally will not grow back the same way even after years of leaving it off.