From Australia a 60 day visa can be extended by 30 days at Chaeng Wattana making 90 days, then a one land crossing such as Aranyapratet to add another 30 days before heading back to Aust for 1 or 2 months. Does this seem to be ok by the new regime of visas?

1 time this should be OK. But no way to tell how they will handle it when you continuously stay 3 months in Thailand and then go back for 1 month for another round.

They might question you and if you can show your income is from abroad and not from within Thailand, you might very well be OK.

You will only get a 15 day entry at a border crossing.

I assume the 100 days you wrote about are how many days you need for your trip not the total days you would get.

Why don't you get a 2 entry tourist visa and skip the 30 day extension and do the border run after the first 60 days that would give you total of 120 days.