Thanks for becoming the advocate for your horses!! Welcome!
I have not written much about feeding the foal though I have mentioned it in podcasts and in replies. I may need to add hash tags for the search engine.
The most important thing about feeding foals is to NOT over feed them. Forage is the basic form of feed and it looks like you are doing that. You did not mention pasture and this is so common an oversight when discussing what horses are being fed. The total forage is about 1.5% to 2.0% of their body weight. Of course weighing how much they consume on pasture is impossible, you can assume several things by the total amount of time on pasture as well as the quality of pasture and the horse density. The assumption here is that pasture in addition to hay (hay, pellets, cubes, etc) will lead to satiation at some point and that the horse will self regulate. In other words, the hay will be left uneaten. There may also be preferences of the horse too (sweet spring grass tasting better than hay or dormant winter pasture less appealing than last summer’s grass in the form of hay).
Salt free choice is always good but you do not need to balance the calcium and phosphorous as there is no grain being fed to make the Ca:P ration imbalanced.
I do not know of “izmine” and I don’t believe you foal will be selenium or Vitamin E deficient because this is rare. If your foal does not have white muscle disease then these 2 should be OK.
SBM should be fed by weight. Just making sure that your “6 oz” is weight and not volume. This is often confused. I recommend 1 pound of SBM for a 1200 pound horse. So that 16 oz. A foal at 300 pounds should be eating 4 oz SBM.
Hope this helps! And the nutrition course will help too!