I finally continued this project after a longer break. And during that break, I realized that having a full-fletched VPC-enabled dev and prod environment can be expensive.
So after receiving quite a few bigger invoices from AWS, I switched my database to RDS, and switched my software stack from Elastic Beanstalk to Lambda. My total costs for up to 100.000 users will now be less than $30 a month.
I also wrote a lot of unit/integration tests. I have so far written 70, and have about 40 remaining until all the critical parts of the system are covered.
It's definitely boring work, but I need to keep grinding until it is done. I think this is also why I had a break. I needed the procrastination.