What is a good tech stack for a startup in 2020?

Frontend should definitely be something along the lines of Typescript and React. Typescript's typing support is a good investment for long term UI dev productivity as the project scales.

Native desktop client could utilize the same stack via Electron.

Backend could start with Python using a popular all-in-one framework like Django to begin with, and scale towards micro-service architecture over time as components grow in complexity.

Heroku is a good all-in-one infrastructure service that takes care of dev-ops for the first year.

This is probably going to be unpopular, but whichever stack helps you ship fastest. If you're deploying microservices as a brand-new startup, you're absolutely doing it wrong.

Like others have mentioned, React+TS on the frontend is a great start. On the backend it depends on how technical your team is but Laravel/PHP or Node+TS is easy.

The best stack is the one you can actually ship fast. If your v1 isn't a messy disaster, then you're moving way too slowly.

Frontend: Typescript/React. Epic CSS and all sorts of beautiful pages and UI parts TailwindUI.com.

Backend: Golang (Go) and a good framework like Gorilla.

Clojure + Datomic & ClojureScript + DataScript