The screen update loop needs to run at least once per 16ms (that is, once per frame) to have smooth 60Hz UI updates.

Animations that run on the coprocessor (fullscreen video and progress spinners) don't require updates from the update loop, and remain smooth even if the update loop is blocked. (So you can use AVI video playback or the builtin progress spinners to entertain the user while doing heavy tasks on the mcu.)

In App.main, create an event slot named either "Idle" or "Update", and connect it to a C function that processes your message queue.

"Idle" will be called repeatedly when the ESD thread is waiting for EVE (frame swap, loading, etc.)
"Update" will be called once inbetween frames.

Depends on your requirements. "Update" is safest.

FlashAddress loading is for loading from the flash that's connected to the BT815 GPU directly, not for CPU flash.

I'm not familiar with STM32. Is the flash directly memory-addressable, or does it go through a specific interface? There's a loading mechanism for loading from memory in Esd_ResourceInfo, but it's not been carried over to Esd_BitmapInfo yet.

REG_CMDB_SPACE is always set to (REG_CMD_READ - REG_CMD_WRITE) & 0xfff.

(REG_CMD_READ - REG_CMD_WRITE - 4) & 0xfff

You can supply the newblob command alongside the update command, to write a specific blob, as follows:
progflash newblob "unified.blob" update "flash.bin"
Can you verify if the blob in your flash binary (first 4096 bytes) is identical to the stock blob that you're writing?

If the flash is blank, the coprocessor can attach it, but will not be able to enter fast mode. You'll be able to read and write to it through the coprocessor, but it will be limited to single channel mode, and will not work from the graphics pipeline. You may need to consult the datasheet of your flash to double check whether the basic commands follow the same de facto standard.

Once you write the firmware blob onto the flash, switching to fast mode should just work. If the unified blob doesn't work, you may want to check with support if they can provide one specific for your model.

You can get a preview of your images using EVE Screen Editor. Load the converted asset using Raw type, and set up and draw the bitmap as you would on hardware. It has a zoom function.

The blob in the flash contains a series of opcodes that control the SPI master when operating in fast reading mode. It also has some verification checksum in it, iirc. It does not seem to be documented, however. The unified blob should work with a large range of flash.