[quake]

I've found Cargo more than up to the task of managing build configurations, and doesn't require monkeying around with CMake scripts or Makefiles. It was pointed out in another comment but you can gate features and crates based on the target you're compiling to. Cargo also supports custom build profiles so you can also pick and choose what you want even if it's all on the same target.

Creating a heap in Rust on a cortex M is safe and cheap-ish with a crate supported by the rust-lang developers. Much easier than implementing your own free() method on a memory pool.

I think you would like rtic. Not a pre-emptive rtos, but a way to manage context between ISR's without relying on some kind of module or global variable that can get corrupted by multiple accessors. Very minimal overhead compared to FreeRTOS