Here's the idea -- with the latest PoD possible, make it so that in the US' century following WWI* no recession (1) lasts more than four years, (2) sees GDP drop no more than half as much as the first part of OTL's GD (1929-33).**
*can be altered, or averted if need be
**I may be wrong, but I believe this was roughly the impact of the post-WWI recession OTL...