Mm I'm not sure if you get picked for flight screening by competitiveness. DFR told me once you pass all the stages leading up to flight screening, everyone gets a go at flight screening and that candidates are picked in the order they complete assessment day (I don't know if this is true. DFR also told me I will sit 3 assessment days and 3 OSBs..)
I was also told there was a huge backlog of applicants waiting to do flight screening from last year (for DEO at least) and that's why the gap between assessment day and flight screening is so long right now.