… I also have not worked out which malts to hold back until the end of the mash and maybe BrewBama will answer that. …
I am sure there’s a much more technical explanation but this is my simplistic version. Here’s how I try to think about it: The mash is a ‘stew’ used to convert starch to sugar. My No 1 goal in the mash is pH and temperature control to aid conversion.
Note: I hot steep dark grains once the main mash conversion is complete. (Many confuse hot steep with cold steep which I do not do.)
Dark, roasted grains and malts—such as chocolate malt, black patent malt, and roasted barley—are kilned at high temp. The high temp destroys the enzymes and burns the starches. So, even if included in the mash they aren’t really being mashed—there is nothing left to convert. The burnt starches and husks can add an astringency plus — and this is the reason I hold them until Vorlauf — they screw with pH.
Crystal malts have been ‘stewed’ prior to kilning so they’ve already been mashed in their husks. The starches have been converted to sugar ‘crystals’. So, they don’t need to be mashed either. …but the lighter colors probably wouldn’t hurt pH. I routinely hold all C-Malts back but anything over ~80° definitely gets held back.
However, ‘character malts’ like Oats, Wheat, Rye, Amber, Brown, Victory, Melanoidin, Biscuit do need to have their starches converted. So, they’re added to the main mash.
Once conversion is complete and I am no longer concerned about pH, I increase to mash out temp and add dark grains for 30 min.
This way, every mash is the same: base malts plus character. It simplifies mash pH control.