Your software engineers will be expected to write software.. you might need to find some different ones
FOTA, OTA, and IAP are materially the same thing.
Use whatever means you choose to implement these things, perhaps implement several. Use the protocols and methods to pull the data.
People use HTTP, to pull .BIN or .HEX, or compound files across networks. They use FTP. Some Cellular modems provide download methods, or socket methods, independently of you using PPP via a Data Mode connection.
On serial ports X-MODEM and Y-MODEM are popular due to the protocol being standard, and supported in a wealth of terminal applications spanning 4 decades.
Use MicroSD cards to hold updates, use USB MSC to read sticks, and allow drag-n-drop.
Use USB DFU methods.
Stage data locally in RAM, SDRAM, QSPI NOR FLASH, etc so they can hold complete/intact firmware images.
People use compression and encryption to protect things, sign with elliptic curve methods.
If free stacks don't come with what you need, perhaps look at commercial ones.
The more methods you provide/allow for the more future proof and flexible you can be.
