what do you know? use that. Also you could start with reddit's latest opensource version.
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
Throwing Mongo at the problem is not really a solution.
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.