STM32s have great Rust support[0] if that matters to you. Personally I strongly prefer the embedded_hal[1] and RTFM[2] APIs over the Arduino ones, although the peripheral coverage is a fair bit weaker. Their DISCOVERY kits[3] (preassembled boards that are ready to program, just like Arduinos) are ~$20, and also double as programmers if you eventually end up designing your own boards.

[0]: https://docs.rust-embedded.org/discovery/index.html

[1]: https://github.com/rust-embedded/embedded-hal

[2]: https://github.com/japaric/cortex-m-rtfm

[3]: Such as https://www.st.com/en/evaluation-tools/stm32f3discovery.html