One medium-sized server can now easily serve the entire banking system of a large country. So you don't need clusters of servers with careful tradeoffs between consistency and availability because of the CAP theorem.
Just get a big honking host, slap Postres on it, and run your application server in a VM on the same host. For backup, export the WAL logs from Postgres onto your favorite off-site blob storage. UI for it? There's Pigsty.
It still makes sense to host this in a cloud provider, just because you probably don't want to deal with actual hardware. And maybe get a second standby server for redundancy.
That's it.
You'll need a bunch of external dependencies for email, 2FA, observability, LLMs, and so on. But they should all be outgoing dependencies that should not allow access back into your system.
Do you think you can hit these load levels? For compute, a 64-core CPU can saturate a 10G network with simple template rendering. If the code is written in something reasonably fast (Go, Rust, Java).
Of course, if you're on a scale of Amazon or Stripe, you'll need more than that.
Sometimes I do miss owning the hardware to run a thing.
What I don't miss was having all of those appliances run a different base distro of Linux, each with varying levels of update support, while the security team keeps reading security scans to me.