What is the best tech stack to use for creating 'community sites' like Clarity.fm?

There's no "best" answer here, but you can approach it a number of ways.

For trendy/future-facing, an isomorphic JS stack (Redux, React, etc.) could be a good move. The architecture is scalable and — if no one on your team hacks around the implementation details — it's easy to maintain (and onboard new team members).

You can also use WordPress as your CMS, but that can get weird when scaling, and isn't the most developer-friendly solution due to WP's wonky architecture. You can unfuck it somewhat using something like roots.io (Trellis/Bedrock), but at the end of the day it's still WP, and those quirks are part of the price you pay for the ease of starting up.

Another option is to use WP as a CMS, but read the content from the REST API and generate sites with other front-end tech. I recently built some tools to generate static sites (for deployment in Amazon's S3/CloudFront) that pull the WP data as JSON and then generate static pages with my preferred front-end tools, leaving the weird PHP templating out of it entirely.

To take a crack at a tl;dr: talk to your development team about how you want this to scale, and where you see it going in the future. If you don't have a development team that can make those calls, talk to a specialist to help lay out a roadmap.

If you need help with this, I'd be happy to take a look at what you're planning and offer some insight into how it could be built to scale and grow painlessly (or at least as painlessly as possible). Send me a message and we can talk details.

Good luck!