Resolved on my own.
https://shopify.dev/docs/themes/pricing-payments/purchase-options
has a little note that says merchants must meet eligibility requirements, which links to this:
Only there does it say that you MUST have Shopify payments enabled. Turned on Shopify payments in the development store and everything started working instantly.
Wish there had been some indication/error/feedback so I didn’t have to spend so long needlessly hunting. Could have easily returned a graphql error when trying to create the plans.