Migrating to Perth with primary school children

We are a Swiss family with two kids (9 and 11). We used to live in Perth between Protected content Protected content moving back to Switzerland.
Now we are contemplating moving back to Perth, but have some reservations about the impact on the kids regarding school and grades (and future jobs).
Summed up, the main question would be: "If the kids are thrown into a higher grade in Australia, will they have enough time and support to catch up to all the other kids before it starts to hurt their chances at life with a good job?"

Has anyone here made this step and been in similar shoes? Could anyone share some insight into their journey with the WA school system?
We have compiled a list of relevant facts about the kids and the questions that we are trying to answer. See below if you feel like helping.

Any help would be appreciated.

Facts about the kids
- School year in Switzerland ends in July and starts again mid-August
- Our daughter is currently in grade 5, will have started grade 6 in December Protected content
- Our son is currently in grade 3, will have started grade 4 in December Protected content
- In WA, our daughter would start year 7 (straight to high school) in Feb Protected content
- In WA, our son would start year 6 (!) in Feb Protected content of being right on the cut-off date (June 30)
- Both kids have attended weekly English classes for the last 6 years here in Switzerland
- Our daughter’s English is quite good, but obviously missing lots of vocab
- Our son’s English is basic. He understands a lot but doesn’t talk much

- Looking at the curriculum, it seems that Western Australia not only starts at an earlier age with school, but also is a bit further ahead with at least some subjects (e.g. Maths)

- Both kids are keen on moving to Australia again and like going to school in general
- Both kids’ grades here in Switzerland are fantastic (so they are generally very capable)

Questions

Related to Enrollment
- We wouldn't live at the final address until April Protected content . (We do have proof of ownership of a house. Currently other tenants are occupying the house until March Protected content
- What does that mean for enrollment for the Protected content year? Can we still apply for local intake into the public school? Or how does it work?
- If we moved e.g. in Nov Protected content , would we have to enrol the kids for school for just a few weeks until the school year ends?

Related to grade selection
- Could we start the daugther in year 6 (rather than straight to high school)?
- We would hope to start the son in year 5 (one year back) if there is no disadvantage for him stemming from this. The request would likely be granted since his birthday is right on the cut-off date (according to the principal of the local school). Or would it be even possible to start him in year 4 because of the language barrier?
- What is the normal process for kids migrating from overseas with different mother tongues, besides Intensive English Classes? Would there be special tasks / homework for them to help catch up on topics they have not yet learnt in the schools abroad?
- What is the grading like in primary school? Can you “fail” subjects and get stuck in the system?
- How flexible is the system when it comes to which grade they get into? The principal of the local school seems convinced that it’s very, very hard to get around the fixed ages and cut-over dates and it would need official requests to the government for exceptions.
- How would the schools deal with helping them to catch up on topics they are not yet familiar with?
- How much time would they have to "catch up on things" until they are considered a lost cause? We will certainly help at home, even organise private lessons etc. if necessary.
- Do the kids’ grades in primary school have a significant impact on their chances in high school / university later on?