XMPP was supposed to be this. There was a boom in the early to mid-2010s (I worked at HipChat back then, and it was built on top of XMPP). IMO, it works well; it scales, but none of the commercial solutions picked up for whatever reason. In the end, as you say, proprietary protocols like WhatsApp or Slack won the market.
I've heard great things about Matrix. Peeble's founder, Eric, was building Beeper, a chat app with a similar purpose to what you describe (it recently got acquired by Wordpress). I believe that the core was built on top of Matrix, so you might be up to something there.
I'd highly discourage you from using SMS; it's very insecure. I'd go as far as to recommend you not to implement your own auth and instead use something like Auth0, WorkOS, SuperToken, or SSOReady (https://github.com/ssoready/ssoready), among others.
Building auth stacks is not trivial and is not what will make your SaaS successful. The more you can leverage experts to focus on what makes your SaaS special, the better.
Every time I read about Nomad, I wonder the same. I swear I'm not trolling here, I honestly don't get how running Nomad is simpler than Kubernetes. Especially considering that there are substantially more resources and help on Kubernetes than Nomad.
I agree with @metaltyphoon on this. Even for small teams, a managed version of Kubernetes takes away most of the pain. I've used both ECS+Fargate and Kubernetes, but these days, I prefer Kubernetes mainly because the ecosystem is way bigger, both vendor and open source. Most of the problems we run into are always one search or open source project away.
The most significant value I get from Costco is that the quality is consistently high and the options are well curated. I like not having to go through 100s of brands of basic staples like yogurt, toilet paper, milk, etc...