What is best stack for creating a medium size web app as light weighted as possible?

I like the "ReactJS" approach, which has devs build out the "view" (UI) first, followed by logic... leaving data handling for the end.

I say this because I assume you are learning ad you go. So I suggest using simple HTML/CSS and build out your mock-up . If you have some experience, then lean into libraries like "Bootstrap" to save time while mapping out your features. After you get a draft going... Focus on how info/data will be handled. Do you need third party apis? Will there be a lit of data to save? Are you familiar with SQL/No-SQL ?

If you just want someone to make up your mind so you aren't wasting time on trying to understand how to choose a language... I would suggest using mature and current libraries with big communities: Setup an en irinment locally using NodeJS .for your server-side language. This will allow you to use JavaScript on the client/view (frontend) side AND on the backend (Because NodeJS uses JavaScript for creating code on the server and view removing complexity of learning another language) Depending on what you expect "fast" to be, if you won't be changing the UI much (static web site) just use HTML/CSS and be done... Static websites are super fast because they don't require any data .. but if you know your app will be delivering dynamic data alot, then learn about "caching" and progressivecweb apps.. a light weight library is "Preact".. which is popular. Hope this helps.

The real answer is, you can use what you want, 20 users is nothing, if you are doing it from scratch it’s going to use very little RAM, you can look in to headless solutions you can adopt to speed up coding. Keep in mind most solutions are not heavy because the tech stack but because the features they provide, so start and see what you are going to need to code and think of time to market now, don’t pre optimize

I'm building something similar with MEVN (with MySQL, not MongoDB). Took me a really long time to figure that it was right for me. I started learning in this order:
MySQL > Node.JS > Express > Vue.JS. If you're not ready to learn a framework like React, Angular, Vue, or Svelte yet, you can just use the MEN stack (MongoDB, Express, EJS, Node.JS).

Honestly if you seek for something "light weighted" I would use Golang for the job because it compiles to the binary (with all the dependancies) so you can simply copy binary and start it from anywhere without worrying about performance because it has extremply low memory footprint and great performance (almost as C++). Looking into Go and investing some time in it is smart decision, you wouldn't regret it and you would learn something new. If you don't intend to use too many libraries (datepickers and stuff like that) then I would use preact, but if you need those kind of libraries then go with the react. But then again if you are not ready to learn something new stick to the technology you know the best.