Remember that "modern" is constantly changing. Whatever "modern" thing you pick is going to be either uncool or defunct tomorrow. Your choice today saddles developers for the rest of the project's lifetime.
Which is fine. There's every reason to think that Go and Rust will be in the "uncool" category rather than the "defunct" category. If they match your requirements and you can assemble the team to use them, they're both fine choices.
But "not modern" for Java also means "durable". It stands the test of time, just like your code will. (And don't kid yourself -- your "prototypes" will be operational code.)
A lot of the features that make Java "bloated and verbose" are also the ones that make it durable. It's a language designed for large projects -- heavyweight for "hello, world", but prized by coders who have experience in revisiting 10 year old software.
Or you could pick Kotlin, which is basically a cleansheeted Java. It uses all the same libraries and frameworks, which are actually more important. It's got some nice language features, most of which Java is also adopting. Either is a fine choice -- Kotlin is a bit cleaner, Java is a bit easier to hire for. The differences aren't all that big.
I've never regretted picking Java, even before modern tooling (I wrote my first Java with vi, and still do sometimes.) The tooling makes it excellent and keeps improving.
There are some frameworks I regret having forced on me, but they're not Java the language. People who can't tell the difference between the language and the framework are not going to advise you well.
No matter what you pick some things will be easier and some harder, and you won't know which. The worst case is that the language dries up. Avoid that case and the language won't be your worst problem.