Use One Big Server (2022)

Picking an arbitrary price point of $200/mo, you can get 4(!) vCPUs and 16GB of RAM at AWS. Architectures are different etc., but this is roughly a mid-spec dev laptop of 5 or so years ago.

At Hetzner, you can rent a machine with 48 cores and 128GB of RAM for the same money. It's hard to overstate how far apart these machines are in raw computational capacity.

There are approaches to problems that make sense with 10x the capacity that don't make sense on the much smaller node. Critically, those approaches can sometimes save engineering time that would otherwise go into building a more complex system to manage around artificial constraints.

Yes, there are other factors like durability etc. that need to be designed for. But going the other way, dedicated boxes can deliver more consistent performance without worries of noisy neighbors.

I am not even thousands km near the level of what you are doing, but my client was paying $100/m for an AWS server, SQS and S3 bucket, for a small PHP based web application that uses Amazon Seller API, Keepa API for the products he ships. Used MySQL for data storage.

I implemented the whole thing in Python, Django, and PostgreSQL (initially used SQLite) put it in a $25/m unmanaged VPS.

I have not got any complaints about performance, and it's running continuously updating product prices, details, processing PDF invoices using OCR, finding missing products in shipments, while also serving the website, and a 4 core server with 6GB RAM is handling it just fine.

The load is not going to be so high to require AWS and friends, for now. It's a small internal app, probably won't even get over 100 users, and if it ever does, it's extremely simple to migrate, because the app is so compact, even though not exactly monolithic.

And still, it probably won't need a $100 AWS server, unless we are scaling up much larger.