if the service just does what you described. I would use GO or Node.
I love Rust but it gives you way more options and GO is straight forward esp using the http library already unleashes a HA web-server handling 155l~ requests a second by just using a small binary. (the footprint is way bigger than Rust, but lets go for low-hanging fruits before you want to think about memory models.
I don't understand mongoDB unless you have a lot of writes, because reads are easy for mysql with writes you can think of switching to tokuDB for higher insert rates ... personally I think what you're trying sounds like a CQRS approach.
But I don't understand everything you want to achieve, maybe you're storing a lot of documents so MongoDB is obviously a very good choice. But I would have to really look through your application and measure it. :)
But from a pure programming / problem domain point of view with the estimation of language complexity and learning curve to output -> node is a very good choice if you want to have it easy, go if you want to have it a bit more challenging but want higher rewards and rust if you a really willing to walk the extra miles and just use 40MB for doing what other systems do with 400MB ....
Rust should also not have the possible GC bottleneck of GO but 10Mio Requests per day (11.574074074 ~ per second) shouldn't be a problem for GO.