I have a Legacy Promise Pegasus R4 (old as dirt) and got it working for my new Mac Studio (M1 Ultra) with the driver from Promise. Use the Promise.com Support Page, Legacy, Pegasus2 Series, Pegasus2 R4. Download the M1 driver for Mac there:
Pegasus2 Series Driver for Mac
SR3.4.16 only support for Mac M1 and MacOS 11.0 or later.
The README is attached.
Best!
Grooveplace