FreeRTOS is pleasant to use.

I've recently been getting into Rust + Embassy + Probe-rs and in my opinion it's been the best embedded experience by far.

Broad support for many different chips is precisely why Arduino is so bad. It has to check pin numbers against a gigantic table for every gpio call.

You want chip-specific libraries. When the software is designed for the hardware everything works better.

The native AVR and esp-IDF frameworks are very good. There's also micropython and circuit python. I've heard good things, but I don't partake in Python.

Personally I think attempting to provide a cross-platform library for microcontrollers is an enormous mistake. This is not x86, you can't rely on any CPU feature existing, which results in awful branching code in places that in a sane framework is a single instruction updating a CPU register

If you want a "framework", Zephyr is the only thing i can think of, that is somewhat hardware agnostic, have great software packages, and fairly widely used.