We bought one immediately for testing.
https://x.com/zachherbert/status/1816864071276462174My succinct take is that Ledger Stax's 180 degree curved eink display was extremely difficult and expensive to make at scale, without defects, and has a very high defect rate. Ledger likely spent tens of millions of dollars on screen development and has to put aside a very high percentage of screens at production due to fallout. Therefore the 1 year+ delays and price increase to $399.
Interestingly enough, the Flex has a much faster refresh rate + input response time due to a likely use of an existing eink panel (no custom tooling, etc). My guess is Ledger will cancel Stax within 12 months.