I've started using what I've dubbed the HAG (HTMX, AlpineJS, Go) stack for my "startup". It's really simple and easy to get into. I do try to minimize the use of third party packages/modules to reduce my attack surface.
For the rest of your questions,
- Auth: Auth0. Simple to get started, and depending on your plan you can enable SSO for business clients.
- Frontend: Plain ol' HTML, CSS, and minimal JS. This is where HTMX and AlpineJS comes into play. Elements can be updated without a whole page reload creating a better user experience.
- Backend: Go. A language I love to use. Fast, simple, and deployed very quickly.
- Database: Turso. Dead simple, easy to use, and with easy replication to different regions.
My app code is all contained in a monolith, which is deployed onto a dedicated web server. I can either update the server resources or using a load balancer I can deploy more servers running the application to increase scale (if necessary).