Whilst my immigration stamp in my passport is valid until September, my multiple non-imm O/A visa is due to expire in July already. Is there any need for me to consult with the IO before September for my extensions of stay - or anything else (other than 90 days reporting)? Thanks!

Nothing you need to do at immigration until September other than 90 day reports after July 31st.

Maybe if you get lucky you will be able to leave and re-enter the country prior to it expiring to get a new one year entry.