I'm wondering if your experience with a custom built bike vs. a mass produced one from a reputable manufacturer, justifies the extra cost.
I have a custom "sport touring" bike, although it's not the one I take for most tours. I got the custom bike with S&S couplers specifically so I could take it on business travel, when I expected 4-6 2-week trips per year. I could take that bike as my second checked bag, free. That lasted two years, until I changed jobs and the airlines got even worse and started charging $$ for bags.
That was my reason for buying a custom bike; what's yours?
The $1500 production bikes for loaded touring are very good. Duplicating that in a custom bike costs roughly $3,000 and up. Unless there's something you really need different, that's $1,500 freely spent with little or no return. Remember the $1,000 app you could get for your cell phone a few years back that did nothing, but you could show it to people to brag about how you could spend that kind of money?
Now for the flip side of the coin, so to speak. If you have problems getting fitted to a bike, that's a good reason to go custom. If you have at least, let's say, a month of touring under your tires, then you might have some ideas of something that's not available in a mass produced frame, that would be another good reason to go custom. If you're going to do lots of air travel and want something to take with you, that's another reason. Just be aware that you might save $300 per trip, and it'll probably cost you $1,500-$2,000 above the "base" custom price to get set up with a coupled bike (or Bike Friday), so it's going to take a LOT of travel for that option to pay off. (Remember you'll want a case, and couplers, and your choice of several other parts will be limited to travel with that bike.)
Let's say you got that custom bike. What do you do with it when you're on tour? If you lean it up against a wall, the wall might scratch that rad paint job. If you run into a convenience store for a nature break, are you going to worry about will the bike be there when you come out? Brag about how much it costs, and do you really want to camp in the park in that town and leave Precious Bike outside your tent? What will you do if it's stolen or damaged? If it was a production bike, it's feasible to budget a couple thou to replace the bike and keep touring. If it takes 6 months to get a new custom, are you going home to pout until next year?