Yes - the 12 month validity commences on the date of issue and not the date of arrival. I have the OA visa from the end of February, but won't be arriving in Thailand until early April. Therefore my renewal date will be end of February 2023. I, too, am coming from Australia.
I have recently finalised my 12 month insurance policy with Pacific Cross, and yes it took the best part of 2 weeks to process all the documentation including my 13 pages of medical test results (I am 70 with a few pre-existing medical conditions and therefore exclusions), policy offer & acceptance, payment authority, insurance certificate & insurance policy.
There were a few hiccups along the way but the overall service and communication was helpful, friendly & respectful.
Just remember, multi-national health insurance providers won't go broke!!!!!