How to choose a platform (CMS) to create a site / landing?

I would recommend a headless CMS solution with the caveat that it requires some Frontend knowledge (like GatsbyJS). Your CMS would be where you store all your data and edit it, and then the frontend would just read that data in and spit out a static site.

If you go the headless CMS route, CosmicJS is pretty good. As is NetlifyCMS (here's a good guide on integrating the latter with GatsbyJS.)

Lastly, my biased opinion is to use Midtype because it's a lot more versatile than a CMS would be, and given that you're building a SaaS product, it can probably do a lot of the basic stuff you need out of the box. The disclaimer is that it won't be as easy to write/edit stuff as it would be with a CMS that has a robust editing UI like CosmicJS or Netlify CMS. If you're interested in learning more about Midtype, hit me up! I can explain more and give you a good deal to test it out.

If you decided to go with WordPress, you can check http://thetheme.io/thesaasx/ which I built for this purpose.

If you liked the design, I suggest you to try the page builder to see how easy it is in terms of developing a landing page: http://thetheme.io/thesaasx/try-page-builder/gutenberg/

You can take a shot at using our service, Caravel Studio, www.caravelstudio.io
We don't have templates yet (on the way) but you can create as many pages as you want, stylesheets, and deploy to the server.
Full control on the design, out-of-box workflow for embedding forms into pages.
Paid project plan ($12/month) adds a custom domain name, and allows to download the final app and all pages/styles, and import 3rd party stylesheets (bootstrap 4 is already provided for all the plans).

How about carrd? https://carrd.co

Webflow is pretty amazing. You have to learn a little at first, but it ultimately is the best bridge between programming and non-programming. It's a drag-and-drop builder, but it exports solid code if developers need to take it over or use it somewhere else. They have a whole site to learn it, if you want to check it out- university.webflow.com

I'm not a huge fan of Wordpress (usually fairly bloated and I am not in love with PHP) but in this case it might serve you very well. Especially with WooCommerce as a plugin for your catalog requirements. I run a half-dozen sites on WP and would recommend it in this case over most alternatives (Shopify is also an excellent platform but it sounds like you guys are mostly devs, and would be frustrated at the lack of configurability).

What do you mean by catalog?