Its overwhelming isn't it. Trying to deal with all your kids different personalities, ages & needs all at the same time!

DD2 is SPD/ADHD. She's really struggling right now. The OT we saw did a full assessment, but we really didn't get much more out of her. I need to sort something else out, soon.

You'll figure out what works & what doesn't in your own time. You just need lots & lots & lots of patience!

In regards to OT sessions, ask for a mental health care plan, or an enhanced primary care plan (EPC). With the EPC you can get 5 subsidised sessions a year with an OT or other regular specialist (psycologist etc) & with the mental health care plan you can get 12. Thats 17 sessions that are about half the full price. Then you can claim back what you do pay on Medicare as well. Mine worked out to be $20 a session in the end, after the initial assessment costs. This was in NSW though, so not sure what would be on offer in SA. I'm sure they'd have something similar.
Have you tried Early Childhood Intervention? Also a NSW thing, but they should have something similar in SA.

Here's the Autism SA page. Not sure if you're close to them, or already in contact, but the number on the page may be of use to you.

You might be able to apply for centrelink benifits to help with the costs of sessions too. Carer's allowance or payment.