I don't disagree with anything you've said, but think there is more going on with your points than a series of signs SA is stagnating behind the other states.
Disclaimer: I know nothing about economics and only hear how people casually talk about SA.
Shop vacancies are definitely noticeable. Just last night I was in Rundle Street for the first time in ~18 months and can't believe how much it has changed. A lot of shops relied on city workers that have dramatically dropped from COVID, and some changes will be permanent as we've all learnt a lot about how we want to work/commute as a result. Even more permanently, pockets of the city are gradually shifting from boomer to international student markets (although obviously that is stagnant right now). I think I also read recently that commercial landowners have some incentive to keep their shop rents unreasonably high, as it inflates the value of their building assets for credit leverage. This can happen to the point where there is more incentive to have vacancies at higher rents than tenants at lower (actually realistic) rents. I think /r/Adelaide talked about this before. But my memory on this is very vague and maybe someone who knows what they're talking about can chime in.
Job opportunities have always been better on the East Coast. I don't think this is a recent thing or even necessarily a bad thing. They simply are bigger and have more diverse economies. I wonder if the recent difficulty is not exclusive to SA but rather everywhere. It's not Adelaide slowing down, it's everywhere slowing down due to COVID and possibly other market cycles. From personal experience, when you do land a job, Adelaide is the place to be. Making ends meet and keeping stress to healthy levels is way easier here. Harder to find a job, but way easier to live with a job.
I know this doesn't really help your situation except to say things aren't necessarily as gloomy as they appear on the surface right now. Most of us have been through the grind of job-hunting at some stage of our lives and it simply sucks very very much.