I can use Ruby on Rails but I don't think an RoR app can handle thousands of comments per minute in an optimised way. I run a midsize app in Rails 5 and already having lot of scaling issues.
Iirc Reddit stopped open sourcing their code from 2017 and I guess understanding and running that 12 year old spaghetti code wasn't worth it anyway. I don't see anything useful on their github page too https://github.com/reddit
Rails will handle "thousands of comments" just fine. If you are hitting scaling issues with a midsize Rails app you probably could look into how to optimize.
Throwing Mongo at the problem is not really a solution.
Worst possible advice, if you pardon my borderline rudeness. That's what most programmers do, following famous saying that for someone with a hammer every problem looks like a nail.
Ops problem is more architecture and scale, IMO, as opposed to what coding language or JS framework.
I’d start with Postgres on the back end and then work with scale as needed over time. The service can be written in pretty much any language like Kotlin, Java, Python, or even Rust if you want. Personally I’ve never used Swift server side.
IMO the worse option is to try to over optimize and use some shiny technology that you don’t understand how to use.
Iirc Reddit stopped open sourcing their code from 2017 and I guess understanding and running that 12 year old spaghetti code wasn't worth it anyway. I don't see anything useful on their github page too https://github.com/reddit