Start with your ground school studies (distant learning) once you hold a Class One medical.
By mid 2021 a much clearer picture will emerge of hopefully restrictions & tiers reductions.
The clock starts to run at the first sitting of the exams. 18 months.
Personally I would start with the ‘hardest’ exams first. Navigation for most? The ATO learning provider might have a more cunning plan, but that might require increased time & effort once the clock has started just depending on the ‘batting order’ (a cricketing expression for our colonial cousins).