These are my business, I run these:
* PostgreSQL database.
* Go for an API.
* Python + Django for a UI.
* Linode hosting, very few larger boxes rather than lots of small boxes.
These are not my business, I want to pay someone to do these or I want such simplicity that it's a non-issue:
* Grafana Cloud for monitoring and alerts (nearly all Prometheus based, but some log based alerts and log based dashboards).
* A few Excel spreadsheets, which I keep as Excel to enable easier sharing with accountant. Besides, he's probably sharing Word docs back to me.
* Syncthing + a NAS for file storage of the business files and accounts.
* I do all payments via PayPal. I wanted to use Stripe but people really want to pay with PayPal so I centralised there.
* I use Xero to track accounts, assets, etc.
I pay about USD 1k per month in various hosting costs, then the Xero subscription, and an annual subscription for MS Office which I consider part of the accounting costs. I use a local small business accountant who only charges GBP 250 for annual accounts.