His typical diet is Purina senior, beet pulp and/or hay squares, mixed together with warm water (we usually even leave it to soak overnight and during the day after breakfast, he has to eat twice otherwise he starts slimming down because he can hardly eat hay due to his dental issues). He has pasture turnout in the day
Hi, do you mean he is hard fed only twice daily? I'm sure your vet would have mentioned all this already, but I imagine that is at least part of the reason for his colic. Especially over night, if he can't eat hay, you need to either leave him on that pasture if he's managing that OK, or feed him something which will keep him going all night if stabled. Horses aren't built to go hungry for long, have an empty stomach. They also need little & often feeding of rich feeds, as too large &/or infrequent feeds are problematic.
So naturally my paranoid self is trying to cross check all the articles and sources and see
That is far from 'paranoid'. It is RATIONAL behaviour! Unfortunately, many marketing, traditional or otherwise stories abound & are taken as fact by those who don't bother to analyse it. There are many differences of opinions, outdated ideas, etc. Even by supposed 'experts'. So don't take anything on blind faith, but do your own homework, to make the most informed decisions you can on stuff.
I definitely wouldn't be feeding him bran mash daily, once a week at most if not once every few weeks with some chopped up carrots/apples/etc., just as a special soft treat for him
Yes, if you're giving it just as a small treat, a handful of bran mash irregularly is not likely to cause further issues. But if you're planning on feeding more than that, or feeding for other reasons, then you need to feed it little & often, as with other food. Over at least a few meals daily is best. And if just feeding for a treat, and you feed him carrot & apple anyway, why bother with one more treat? Stick to the fruit & veg!
The old idea about bran mashes were that they were good for relieving constipation, and feeding regularly/weekly was often as a preventative for this. However, it's the horse's system being upset by strange feed it's not used to that causes gut problems, often leading also to 'loose stools'. In addition, bran(esp wheat) is very high in phosphorus, which can unbalance the diet dangerously, if not considered as part of well balanced nutrition.