Are these your favorite strings? Like no other strings will do?
Most strings I’ve used can handle a half step down. But I like light and low tension, heck, I’ll run TI jazz flats a half step down. Of course, YMMV.
If every thing went south with just a half step down, I’d take a look down the neck. Is it now slightly back bowed? Simply loosen the truss rod until you get the relief that suits you. You also might need to raise the saddles a bit, depending on your right hand attack.
If the strings are just too loose at a half step down, and you’ve improved the set up, it’s on to different strings. DR makes strings just for downtuning. I haven’t used them, but most DR sets I have tried were long enough to make it on four string longscales like the Dbird.
I really like the DR pure blues, a nickel round that comes in several gauges.
Jason at FretNation is supper knowledgable on strings for longscale Dingwalls.
Payson strings are another great option, and they come in multiple gauges.
Both Payson and FretNation will sell singles as well.