I happened to notice on the Script Debugger forum that a beta version of FastScripts 3 is available for testing:
https://forum.latenightsw.com/t/fastscripts-3-0-sneak-peek/2902
The most significant change for me is that scripts will run in separate processes. I have encounterd one issue with the beta, which I’ve reported, but otherwise it seems to run OK (if a bit slow).