I'm not sure where the idea came from that this is hard to do.
To answer your questions in order:
1) People warned aggressively about lock-in of cloud providers with proprietary extensions and you must have chosen not to listen; so, I have little sympathy.
I'm not saying it's black and white; but I hope that you got your velocity required to hit market faster and have made more than you spent because this is the price you paid; and now to get out you'll have to invest a little time on cleaning house, that's the reality of lock-in.
2) S3 is pretty easy as there are "s3 compatible" FOSS projects; min.io comes to mind, or ceph with a RADOS plugin, or Riak with the s3 plugin... there's also s3proxy with a multitude of backends..
3) Aurora serverless is replaced by knative
4) Beanstalk is just classical servers with an auto-scaler component, auto-scaling depends greatly on your provider, so I can't say how easy or hard it will be, if you're using kubernetes then understanding your load should be easy at least.