When should a couple graduating 1 year apart start applying for BTOs for maximum grants and have a high chance of success?

I'm a Singaporean citizen and my gf is a PR, I'm graduating in May 2020 and she is graduating in May 2021. With just my first job starting income around 5-6k, we should qualify for $35000 of EHG grant, but once my girlfriend starts work after in Jun 2021, our combined income is over $9000 would not be eligible for any grants under EHG. Is income assessment separate from the HDB application itself? If so, when is the date of income assessment? And ideally, when should we start applying for BTO?