Lots of unclear things here
I worry that you are not getting proper advice by this gentleman.
I went with a production bike for my tough tourer, and one thing that stands out is the 35mm tire thing. I've ridden my "expedition bike" on all kinds of surfaces and 35mm tires aren't anywhere near what you want in a bike like you are describing
a custom made frame will cost a lot, save money and look into production bikes. The money you'll save will pay for a dynamo hub etc and way more.
and yes, discs are great with heavy loads and mountains.
a mtb triple is a great setup
a frame that can take at least 50mm tires is a real plus
26in wheels are great and you don't notice wheel diameter when riding a heavy touring bike