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It has an interesting trade-off: no internal flash. But I've found that I add a SPI-flash chip to the MCU in many designs, so it's a reasonable choice.

NXP has rt1020 (Cortex-M7, no flash), or rt1024, which is same but 4 MB flash in the package- it's connected to the SPI-flash controller of the rt1020 core, so I think it's a multi-die package.

Look at the even more capable and comparably priced ESP32-S2 and ESP32-C3.

How fast is the serial interface over USB? (What you need if you want to connect it to a normal PC)

The RP2040 actually has built in 12Mbps USB 1.1, no serial bridge required. But if you do want to use a serial to USB bridge the max baud rate is 921600.