Given the paucity of STM32F423ZHJ6 at the usual vendors, wondering if it possible to use / support other package variants of this chip, especially if only WiFi or BT is desired ? (LQFP64, UFBGA-100, etc)
In theory the 144 pin LQFP would work, but this has not been tested. Smaller variants are not supported as they would require different mappings for things like QSPI.
The 413ZHJ6 can be used via the contract programming house; there are different keys for this but that only affects the SPI image used for initial power up.
Generally availability is improving, though.