I’ve just been through the assessment day process. We couldn’t actually do the sims straight away, as the sim had broken, and they were trying to get more slots/assessors organised to clear the backlog. (I don’t think there was anyone local on my day - one from US, couple flew in from NZ, Perth etc).
But they did say it doesn’t matter when you do your sim with regards to the hold file - it is based purely on the date you attended the assessment centre. And we were on week 7 of an 18 month recruitment process.