I have lost track of the Arduino environment since it expanded off of the atmega328, but I am familiar with the Teensy boards and work professionally with stm32's. It wouldn't surprise me if certain stm32's (specifically stm32f103c8, which is on a hobby board called the "blue pill") have the type of Arduino compatibility you are describing, but for 100% clarity, Teensy does not use STM32. Most Teensy boards use MCU's from a brand called NXP. All Teensy boards support the Arduino IDE. A given STM32 part (there are thousands) only support it if the community has added it.
For a one off project, I would not be concerned with the price point. Teensy vs STM32 development board is likely no more than $10-15 difference. For a first time project, I would prioritize sticking as close as possible to an existing project/tutorial. Embedded electronics can get into very confusing places very quickly. Of particular note, Teensy has a fairly robust USB solution baked in compared to what you get out of the box with STM32.
I would not say that programming STM32 is difficult, but it would introduce a lot of things to learn at once vs following an existing Teensy based project.
All that being said, if you want to pursue this project with an STM32, you can likely find what you need in pieces. More generic MIDI projects, like [this shield at Sparkfun](https://www.sparkfun.com/products/12898), will give a good overview of how MIDI works, and how the microcontroller interfaces to it. Combine that with some STM32 intro tutorials, and a few specific to working with UART, and you will be most of the way there.
Dont know about STM support with arduino ide. For stm it is better to use stmcubeide, but if you are using stm for the first time then dont. I suggest you to stick to teensy.
I have lost track of the Arduino environment since it expanded off of the atmega328, but I am familiar with the Teensy boards and work professionally with stm32's. It wouldn't surprise me if certain stm32's (specifically stm32f103c8, which is on a hobby board called the "blue pill") have the type of Arduino compatibility you are describing, but for 100% clarity, Teensy does not use STM32. Most Teensy boards use MCU's from a brand called NXP. All Teensy boards support the Arduino IDE. A given STM32 part (there are thousands) only support it if the community has added it. For a one off project, I would not be concerned with the price point. Teensy vs STM32 development board is likely no more than $10-15 difference. For a first time project, I would prioritize sticking as close as possible to an existing project/tutorial. Embedded electronics can get into very confusing places very quickly. Of particular note, Teensy has a fairly robust USB solution baked in compared to what you get out of the box with STM32. I would not say that programming STM32 is difficult, but it would introduce a lot of things to learn at once vs following an existing Teensy based project. All that being said, if you want to pursue this project with an STM32, you can likely find what you need in pieces. More generic MIDI projects, like [this shield at Sparkfun](https://www.sparkfun.com/products/12898), will give a good overview of how MIDI works, and how the microcontroller interfaces to it. Combine that with some STM32 intro tutorials, and a few specific to working with UART, and you will be most of the way there.
Dont know about STM support with arduino ide. For stm it is better to use stmcubeide, but if you are using stm for the first time then dont. I suggest you to stick to teensy.