1. elastic beanstalk, no docker: EB comes with really nice defaults so that you can quickly whip up a flask app, upload it and it just works. It provisions a small ec2 instance by default which iirc costs 10ish a month at best. Importantly, any operation you do with EB will by definition be ready for continuous deployment since you don't get an option to ssh into the machine to deploy. It's extensible enough to add whatever extra stuff you need as well. Only thing it can't do is simple caching ( if you scale to more than one instance that is), but that can be solved by having a separate eb deployment for a worker that can take care of all these aux stuff (elasticache is expensive I think). In a pinch, it also scales well (though the default cheap deployment does not have a load balancer).
2. Just suck it up and go with RDS postgres. Again, I see too many things going wrong with spinning up your own db in an ec2 instance especially if that instance goes down. Im too lazy to write backup scripts and keep track of them! The cheapest RDS postgres costs 13 a month or so, but I just suck it up to power whatever side projects I do. Postgres means I'm working with something I know, and I get full text search and pubsub for free. And whatever I write is not locked code in anyform, and can be scaled up if needed as well. More importantly I'm only spending my time in technologies that are relevant for me in my day job so that's a win.
3. Github Actions to deploy to eb. just a few lines of yaml and you instantly get continuous deployment directly from your repo for free! Really can't beat that.
I have meant to try out heroku since it could be cheaper from what I have read. But I couldn't figure out what their S3 alternative is or how different they are from the canonical cloud offerings.
I'm sure it can be so much more cheaper, but I'm not good at advanced networking or sysadmin, and I'm too lazy / bored / disorganised to write deploy scripts or sshing into remote machines. I'm also always afraid of if/how long I need to re-provision a vps/ec2 if it goes down. Not that they do, but they can, and that scares me.