Hey, Ed. You're taking on quite a task for ADS and CDS, and you're doing great. Consider it therapy.

We all get frustrated in this hobby, and hate having to tear stuff out and redo it, but I think it simply comes with the territory.
My method is to think a lot ahead of time, make drawings where appropriate to assure that things will fit, and proceed slowly and deliberately. But that's me, and I'm an engineer (civil/environmental, not loco -- well, maybe a little loco!) by profession, so it's natural for me. It also means that this layout it taking FOREVER to get underway, and the kids are losing patience with me.
Looking at the pics you had referenced on connecting flex track gave me an idea. I still think the couplers are critical to good alignment, but the idea was in staggering the connections. Ideally, you'd want the connections staggered by half a length of flex track, though that would be very difficult to handle and achieve, as you would have to "thread" a free rail through all the ties on the adjacent half. Maybe just several ties or several centimeters would be OK. This is good because it would avoid having the joints at the same place, and should make for smoother operation.
Unfortunately, I've already sized most of my flex track (though have not yet attached it permanently), and just getting it cut to proper length around curves is enough trouble! LOL
Looks like you've got a good thing going. I might suggest using a cork roadbed under the track, however, and that will help give something to attach the track to while you are fiddling with it.