Your bow's main issue has practically nothing to do with long string tillering. At least that isnt the root cause.
It was handicapped before glue up. It simply has too much thickness taper in the limbs. You glued it up with a total of 1/4" of taper. That is a LOT. I would have NOT tapered the belly piece at all, which would have meant half as much, or 1/8" total taper, which would have been plenty... and made those limbs look and act a LOT different than what you're seeing there.
Your extreme amount of taper is evident in the profile differences between the first two pictures. Yes, a long tillering string will tend to flex less of the outer limb, so they appear weaker when the bow is braced with a proper length string, but if the long string was the culprit, in the second picture, with the bow being pulled WITH the long string, the outer limbs should appear strong, but they don't. They already look like they're bending too much... your recurves are already opened up a lot... too much for all the more it's being drawn... and when braced with a shorter string, you lost even more of them. This is because there's too much taper in the limbs allowing the outer limbs to flex too much.
Solution? Tough one. I would either saw off the belly lam and replace it with a parallel piece about 1/4" thick, OR, remove enough wood from the belly piece to MAKE it like a parallel piece. Of course you can't get under the riser with the second option, so that wood would stay there and act like a power lam... though definitely not needed in THIS bow.
I would love to see you saw the belly and handle off, clean it up, and glue on a parallel piece. It would be a completely different bow... and awesome with that unstrung profile you got going on there.