I m facing very similar problems from day one I purchased an nano RP2040.
Tried different (original arduino) RP2040 boards, computers (including Mac), USB cables. Tried different arduino IDEs, mbed OS with no result. No problems with Mega2560 board though. Tried all the described routines, including flashing with blink.ino.elf.uf2 which works in principal, but almost every time I load a new sketch, either the programm cannot be loaded or the USB port connection is corrupted.
I m using Win10, so CH340 should not be the problem, but the PC does not recognize the board whenever I replug ("unknown usb device (device descriptor request failed)").
For linux users, running this post_install.sh file seemed to help in some cases, but I have no idea if and how something simlar could be done on Windows.
Some of the behaviour appears rather unsystemically, so not sure where the problem really is.
This is really annoying, so any help is greatly appreciated.