This is using Deno as the glue between Rust and V8; it leaves many things to the end-user, such as stdlib implementation. This isn't quite the same as building it from scratch, but it's still building a good chunk of the runtime I think.
Javascript. Not to remove it, but to make it better from the start. Current JS feels painful to me because it feels like piling hacks on top of a poorly-designed language to make it palatable. Doing it better from the start sounds amazing.
Peplink seems pretty good; they do have a Cloud:tm: management offering called InControl2 but as far as I'm aware it's entirely optional. I've had good luck configuring everything via the local UI. My setup is a Balance Two + a few One AX APs.