"Whenever I investigate examples, though, I find the devs have to go through a ton of effort to avoid memory allocations"

Yep, also the median dev in a GC'ed language is simply incapable of writing super efficient code in these languages because they rarely have to. You would have to bring in the best of the best people from those communities or put your existing devs through a pretty significant education process that is similar in difficulty to just learning/using Rust.

The resulting code will be very different to what typical code looks like in those languages, so the supposed homogeneity benefits of just writing fast C#/Java when it's needed are probably not quite true. You'd basically have to keep that project staffed up with these kinds of people and ensure they have very good Prod observability to ensure regressions don't appear.