The WeAct v2.0 board (also sometimes known as the "blackpill") is a micropython-capable board based on the STM32F411CE (512kB flash, 128kB SRAM) that costs only a few US dollars and is widely available from various Chinese suppliers. It is a breadboard compatible breakout-style board that has reset/BOOT0/user switches, a user LED and a USB C connector, along with 25MHz and 32.768kHz crystals.

Although the board has an unpopulated flash footprint, I don't have any SPI flash ICs around to test it and add support, so this only supports internal flash for the time being. I have ordered some and will try to test the external flash when I get a chance.