Converted without any code changes into an installable Arduino and PIO library.
All credits go to Flamabalistic
PicoSoftwareSerialExample
Quick 'n' dirty PIO based implementation of SoftwareSerial for an RP2040 based board
What definetly works:
Baud rate of 115200
2 streams running on the one PIO block (taking up all 4 state machines)
Not too sure about the reliability of it all, but it should work in a pinch!
A dedicated implementation of SoftwareSerial for the Pico which uses the PIO. I have tested it with a speed up to 115200 and it works like a charm.