that’s the lore, FP3 being useless for relock (irrespective or on top of of /e/OS images signed by google test-keys) because you can overwrite keys in EDL/QDL mode easily. But as to the flow of the diagram you posted before: you can just flash other warning screens if you’re not an OEM needing the Google stamp (as in, certification).
As to the OP question: there’s a testing channel and links to ota images with A15 for the FP3 and yes, I’d say the delay is always due to capacity