In the beginning my startup only had 2 people. A designer (my friend) and me (a developer).
For our frontend we used Webflow. My friend was able to create the entire marketting site, and all the app UI's without needing help from me. Webflow is an awesome tool for that sort of thing.
For the backend, I built a simple Node/Express API and hosted via Heroku.
To this day, everything is still running fine and the API is processing roughly 200 million requests a month. The total cost to host that on heroku is $50/mo.
You can definitely have a simple stack but have it be highly scaleable!
Currently processing almost 200 million requests a month on a cheap $25/mo heroku dyno.