Oh W'jit it is so nice to hear your plans for DIY! At last, a chance to do things YOUR WAY!! We had a bad experience on full livery with Merrick, which funnily also involved a YO feeding him competition mix

despite being told not to. Grrrrrrrr! We're back on DIY now (different yard) and much happier

Anyway, to answer your question, your planned feed regime is *identical* to Merrick's, and he has thrived for 2 winters on it (aside from the competition mix episode!).
As I'm sure your research has told you, you've accounted for warmth, slow release energy, good quality protein for muscle and condition, and vits/mins. And all on high fibre, low starch

10/10
Alfa-A is sometimes avoided by people because it is molassed, but OH bought a bag by mistake our first winter

and being a penny pincher he ignored my horrified cries and insisted we tried it. Merrick was fine and we used it ever since with no problems. Currently finishing a bag of Happy Hoof (his "summer chaff" so to speak) but will return to Alfa-A as soon as it is finished. Competition mix is invariably molassed too, so if he's been okay on that, the Alfa-A shouldn't cause any problems.